Semiconductor device and method for manufacturing the same

ABSTRACT

A semiconductor device includes a pixel portion having a first thin film transistor and a driver circuit having a second thin film transistor. Each of the first thin film transistor and the second thin film transistor includes a gate electrode layer, a gate insulating layer, a semiconductor layer, a source electrode layer, and a drain electrode layer. Each of the layers of the first thin film transistor has a light-transmitting property. Materials of the gate electrode layer, the source electrode layer and the drain electrode layer of the first thin film transistor are different from those of the second transistor, and each of the resistances of the second thin film transistor is lower than that of the first thin film transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor devices, display devices,manufacturing methods thereof, or methods for using the semiconductordevices or the display devices. In particular, the present inventionrelates to liquid crystal display devices including light-transmittingsemiconductor layers, manufacturing methods thereof, or methods forusing the liquid crystal display devices.

2. Description of the Related Art

In recent years, flat panel displays such as liquid crystal displays(LCDs) have been widely used. In particular, active matrix LCDsincluding a thin film transistor in each pixel have been often used.Further, a display device in which one or both of a source driver (asignal line driver circuit) and a gate driver (a scan line drivercircuit) are formed over the same substrate as a pixel portion hasdeveloped. As the thin film transistor, a thin film transistor includingamorphous silicon or polysilicon (polycrystalline silicon) as asemiconductor layer has been widely used.

Instead of such silicon materials, light-transmitting metal oxides haveattracted attention. For example, In—Ga—Zn—O-based oxides and the likehave been expected to be used as semiconductor materials needed indisplay devices such as liquid crystal displays. In particular,application of the In—Ga—Zn—O-based oxides and the like to channellayers of thin film transistors has been considered. Further, atechnique for improving the aperture ratio with the use of alight-transmitting electrode as a gate electrode, a source electrode, ora drain electrode has been studied (see References 1 and 2).

REFERENCE

Reference 1: Japanese Published Patent Application No. 2007-123700

Reference 2: Japanese Published Patent Application No. 2007-081362

SUMMARY OF THE INVENTION

In general, in a display device in which one or both of a source driverand a gate driver are formed over the same substrate as a pixel portionas a driver circuit portion for controlling a thin film transistor inthe pixel portion, a lead wiring such as a power supply line or a signalline led from an FPC terminal or the like, or a wiring for connecting anelement to a different element (e.g., a wiring for connecting a thinfilm transistor to a different thin film transistor) is directlyextended from conductive layers used for a gate electrode and a sourceelectrode (a drain electrode) and is formed in the same island.Therefore, a wiring for connecting a gate of a thin film transistor to agate of a different thin film transistor (such a wiring is referred toas a gate wiring) is formed using the same layer structure and materialas a gate electrode of the thin film transistor; a wiring for connectinga source of the thin film transistor to a source of the different thinfilm transistor (such a wiring is referred to as a source wiring) isformed using the same layer structure and material as a source electrodeof the thin film transistor; and a lead wiring such as a power supplyline or a signal line is formed using the same layer structure andmaterial as the gate wiring or the source wiring, in many cases.Therefore, in the case where a gate electrode and a source electrode (adrain electrode) are formed using light-transmitting materials, a leadwiring such as a power supply line or a signal line, a gate wiring and asource wiring in a driver circuit portion, and a gate wiring and asource wiring in a pixel portion are often formed usinglight-transmitting materials in a manner which is similar to that of thegate electrode and the source electrode (the drain electrode).

However, in general, a light-transmitting conductive material such asindium tin oxide (ITO), indium zinc oxide (IZO), or indium tin zincoxide (ITZO) has a higher resistance value than a conductive materialhaving a light-blocking property and reflectivity, such as aluminum(Al), molybdenum (Mo), titanium (Ti), tungsten (W), neodymium (Nd),copper (Cu), or silver (Ag). Therefore, when a lead wiring such as apower supply line or a signal line led from an FPC terminal or the likeor a wiring in a driver circuit portion is formed using alight-transmitting conductive material, wiring resistance is increased.In particular, since the driver circuit portion needs to operate at highspeed, when wiring resistance is increased, the waveform of a signaltransmitted through the wiring is distorted, which impairs thehigh-speed operation of the driver circuit portion. Accordingly, it isdifficult to supply accurate voltage and current, so that it isdifficult for a pixel portion to perform normal display and operation.

In contrast, in the case where a gate electrode and a source electrode(a drain electrode) in the driver circuit portion are formed usinglight-blocking materials and the gate wiring and the source wiring arealso formed using light-blocking conductive materials, the conductivityof the wirings is improved. Therefore, it is possible to suppress theincrease in the wiring resistance of the lead wiring such as the powersupply line or the signal line led from the FPC terminal or the like anddistortion in the waveform of the signal in the driver circuit portion.Further, by forming a gate electrode and a source electrode (a drainelectrode) in the pixel portion with the use of light-transmittingmaterials, the aperture ratio can be improved and power consumption canbe reduced.

In addition, in terms of display performance, high storage capacitanceand higher aperture ratio are demanded for pixels. Pixels each havinghigh aperture ratio improve light use efficiency, so that power savingand miniaturization of a display device can be achieved. In recentyears, the size of pixels has been made smaller and higher-resolutionimages have been demanded. However, the decrease in the size of pixelsresults in a large area where a thin film transistor and a wiring areformed in one pixel, so that the aperture ratio of the pixels islowered. Therefore, in order to obtain high aperture ratio in each pixelin a specified size, it is necessary to lay out circuit componentsneeded for the circuit structure of the pixel efficiently.

Further, a thin film transistor including a light-transmittingsemiconductor layer tends to be normally on and the threshold voltage ofthe thin film transistor is unstable; thus, it is difficult to performhigh-speed operation particularly in a driver circuit portion.

It is an object of one embodiment of the present invention to reduce themanufacturing cost of a semiconductor device.

It is an object of one embodiment of the present invention to improvethe aperture ratio of a pixel portion.

It is an object of one embodiment of the present invention to make apixel portion have higher resolution.

It is an object of one embodiment of the present invention to improvethe operation speed of a driver circuit portion.

It is an object of one embodiment of the present invention to improvethe reliability of a semiconductor device.

One embodiment of the present invention is a semiconductor device whichincludes a pixel portion having a first thin film transistor and adriver circuit portion having a second thin film transistor, or amanufacturing method of the semiconductor device. A gate electrode (alsoreferred to as a gate electrode layer), a source electrode (alsoreferred to as a source electrode layer), and a drain electrode (alsoreferred to as a drain electrode layer) of the first thin filmtransistor have light-transmitting properties. The resistance value of agate electrode layer of the second thin film transistor is lower thanthat of the gate electrode layer of the first thin film transistor. Theresistance value of a source electrode layer of the second thin filmtransistor is lower than that of the source electrode layer of the firstthin film transistor. The resistance value of a drain electrode layer ofthe second thin film transistor is lower than that of the drainelectrode layer of the first thin film transistor.

As an oxide semiconductor used in this specification, a thin film of amaterial expressed by InMO₃(ZnO)_(m) (m>0) is formed, and a thin filmtransistor including the thin film as an oxide semiconductor layer isformed. Note that M denotes one or more metal elements selected from Ga,Fe, Ni, Mn, or Co. As an example, M might be Ga or might be Ga and theabove metal element other than Ga, for example, M might be Ga and Ni orGa and Fe. Further, in the oxide semiconductor, in some cases, atransitional metal element such as Fe or Ni or an oxide of thetransitional metal is contained as an impurity element in addition tothe metal element contained as M. In this specification, among oxidesemiconductor layers whose composition formulae are expressed byInMO₃(ZnO)_(m) (m>0), an oxide semiconductor which includes Ga as M isreferred to as an In—Ga—Zn—O-based oxide semiconductor, and a thin filmof the In—Ga—Zn—O-based oxide semiconductor is referred to as anIn—Ga—Zn—O-based non-single-crystal film.

As a metal oxide used for the oxide semiconductor layer, any of thefollowing metal oxides can be used in addition to the above metal oxide:an In—Sn—Zn—O-based metal oxide; an In—Al—Zn—O-based metal oxide; aSn—Ga—Zn—O-based metal oxide; an Al—Ga—Zn—O-based metal oxide; aSn—Al—Zn—O-based metal oxide; an In—Zn—O-based metal oxide; aSn—Zn—O-based metal oxide; an Al—Zn—O-based metal oxide; an In—O-basedmetal oxide; a Sn—O-based metal oxide; and a Zn—O-based metal oxide.Silicon oxide may be contained in the oxide semiconductor layer formedusing the above metal oxide.

The oxide semiconductor is preferably an oxide semiconductor containingIn, more preferably, an oxide semiconductor containing In and Ga.Dehydration or dehydrogenation is effective in obtaining an i-type(intrinsic) oxide semiconductor layer.

Note that in this specification, a semiconductor device refers to alldevices that can function by utilizing semiconductor properties, anddisplay devices, semiconductor circuits, and electronic devices are allsemiconductor devices.

In one embodiment of the present invention, the operation speed of adriver circuit and the aperture ratio of a pixel portion can beimproved. In addition, in one embodiment of the present invention, thenumber of manufacturing steps can be reduced, so that manufacturing costcan be reduced. Further, in one embodiment of the present invention, apixel portion can have higher resolution. Furthermore, in one embodimentof the present invention, the reliability of a semiconductor device canbe improved.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1A is a top view of a semiconductor device according to oneembodiment of the present invention, and FIGS. 1B and 1C arecross-sectional views of the semiconductor device;

FIG. 2A is a top view of a semiconductor device according to oneembodiment of the present invention, and FIGS. 2B and 2C arecross-sectional views of the semiconductor device;

FIGS. 3A to 3D are cross-sectional views illustrating a method formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIGS. 4A to 4D are cross-sectional views illustrating a method formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIGS. 5A to 5D are cross-sectional views illustrating a method formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIGS. 6A to 6D are cross-sectional views illustrating a method formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIGS. 7A to 7C are cross-sectional views illustrating a method formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIGS. 8A to 8C are cross-sectional views illustrating a method formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIGS. 9A to 9D are cross-sectional views illustrating a method formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIGS. 10A to 10D are cross-sectional views illustrating a method formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIGS. 11A to 11D are cross-sectional views illustrating a method formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIGS. 12A to 12E are cross-sectional views illustrating a method formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIG. 13A is a top view of a semiconductor device according to oneembodiment of the present invention, and FIG. 13B is a cross-sectionalview of the semiconductor device;

FIG. 14A is a top view of a semiconductor device according to oneembodiment of the present invention, and FIG. 14B is a cross-sectionalview of the semiconductor device;

FIG. 15A is a top view of a semiconductor device according to oneembodiment of the present invention, and FIG. 15B is a cross-sectionalview of the semiconductor device;

FIGS. 16A-1 to 16B-2 illustrate multi-tone masks which can be used inone embodiment of the present invention;

FIG. 17A is a top view of a semiconductor device according to oneembodiment of the present invention, and FIG. 17B is a cross-sectionalview of the semiconductor device;

FIG. 18 is a cross-sectional view of a semiconductor device according toone embodiment of the present invention;

FIG. 19 is a cross-sectional view of a semiconductor device according toone embodiment of the present invention;

FIGS. 20A to 20C are cross-sectional views of semiconductor devicesaccording to one embodiment of the present invention;

FIG. 21A is a top view of a semiconductor device according to oneembodiment of the present invention, and FIG. 21B is a cross-sectionalview of the semiconductor device;

FIGS. 22A and 22B are circuit diagrams of semiconductor devicesaccording to one embodiment of the present invention;

FIGS. 23A and 23B illustrate electronic devices including displaydevices according to one embodiment of the present invention;

FIG. 24 illustrates an electronic device including a display deviceaccording to one embodiment of the present invention;

FIGS. 25A, 25B, 25C, 25E, and 25F are circuit diagrams of semiconductordevices according to one embodiment of the present invention, and FIGS.25D and 25G are timing charts of the semiconductor devices;

FIGS. 26A, 26B, 26C, 26D, 26E, and 26G are circuit diagrams ofsemiconductor devices according to one embodiment of the presentinvention, and FIGS. 26F and 26H are timing charts of the semiconductordevices;

FIGS. 27A to 27F illustrate potentials of display elements of asemiconductor device according to one embodiment of the presentinvention;

FIGS. 28A to 28C illustrate display screens of a semiconductor deviceaccording to one embodiment of the present invention;

FIGS. 29A and 29B illustrate electronic devices including displaydevices according to one embodiment of the present invention;

FIGS. 30A and 30B illustrate electronic devices including displaydevices according to one embodiment of the present invention;

FIGS. 31A and 31B illustrate electronic devices including displaydevices according to one embodiment of the present invention; and

FIG. 32A is a top view of a semiconductor device according to oneembodiment of the present invention, and FIG. 32B is a cross-sectionalview of the semiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings. Note that the present invention can beimplemented in various ways and it will be readily appreciated by thoseskilled in the art that modes and details of the present invention canbe changed in various ways without departing from the spirit and scopeof the present invention. Therefore, the present invention should not beconstrued as being limited to the following description of theembodiments. Note that in structures described below, the same portionsor portions having similar functions are denoted by common referencenumerals in different drawings, and detailed description thereof will beomitted.

Embodiment 1

In this embodiment, a semiconductor device according to one embodimentof the present invention is described.

The structure of the semiconductor device according to one embodiment ofthe present invention is described with reference to FIGS. 1A to 1C andFIGS. 2A to 2C. FIG. 1A is a top view illustrating an example of asemiconductor device of this embodiment (specifically, illustrating adriver circuit portion). A cross section A-B in FIG. 1B is across-sectional view taken along line A-B in FIG. 1A. A cross sectionC-D in FIG. 1C is a cross-sectional view taken along line C-D in FIG.1A. FIG. 2A is a top view illustrating an example of the semiconductordevice of this embodiment (specifically, illustrating a pixel portion).A cross section E-F in FIG. 2B is a cross-sectional view taken alongline E-F in FIG. 2A. A cross section G-H in FIG. 2C is a cross-sectionalview taken along line G-H in FIG. 2A.

As illustrated in FIGS. 1A to 1C and FIGS. 2A to 2C, the semiconductordevice of this embodiment has a structure where a driver circuitincluding a first thin film transistor and a pixel portion including asecond thin film transistor are formed over the same substrate. Thesemiconductor device illustrated in FIGS. 1A to 1C and FIGS. 2A to 2C isdescribed below.

FIGS. 1A to 1C illustrate part of the driver circuit portion. The drivercircuit portion illustrated in FIGS. 1A to 1C includes a gate wiring anda storage capacitor line which are provided in a first direction, asource wiring which is provided in a second direction, which isdifferent from the first direction, so as to intersect with the gatewiring and the storage capacitor line, and a thin film transistorprovided around a portion where the gate wiring and the source wiringintersect with each other. FIGS. 2A to 2C illustrate part of the pixelportion. The pixel portion illustrated in FIGS. 2A to 2C includes a gatewiring and a storage capacitor line which are provided in a firstdirection, a source wiring which is provided in a second direction so asto intersect with the gate wiring and the storage capacitor line, and athin film transistor provided around a portion where the gate wiring andthe source wiring intersect with each other.

A thin film transistor 130A provided in the driver circuit in FIGS. 1Ato 1C is a channel-etched thin film transistor. Over a substrate 101having an insulating surface, the thin film transistor 130A includes astack of conductive layers 107 a and 110 a having a function as a gateelectrode layer or a gate wiring; a stack of an insulating film 111having a function as a gate insulating layer, a semiconductor layer 113a including a channel formation region, and conductive layers 119 a and120 a having a function as a source electrode layer or a source wiring;and a stack of conductive layers 119 b and 120 b having a function as adrain electrode layer.

The conductive layer 110 a is provided over part of the conductive layer107 a. The area of the conductive layer 110 a is smaller than that ofthe conductive layer 107 a. In addition, a conductive layer 110 b isprovided over part of a conductive layer 107 b. The area of theconductive layer 110 b is smaller than that of the conductive layer 107b. In other words, end portions of the conductive layer 107 a protrudefrom end portions of the conductive layer 110 a, and end portions of theconductive layer 107 b protrude from end portions of the conductivelayer 110 b. Further, the area of the conductive layer 107 a and thearea of the conductive layer 107 b are larger than the area of theconductive layer 110 a and the area of the conductive layer 110 b,respectively.

The conductive layer 120 a is provided over part of the conductive layer119 a. The area of the conductive layer 120 a is smaller than that ofthe conductive layer 119 a. In addition, the conductive layer 120 b isprovided over part of the conductive layer 119 b. The area of theconductive layer 120 b is smaller than that of the conductive layer 119b. In other words, end portions of the conductive layer 119 a protrudefrom end portions of the conductive layer 120 a, and end portions of theconductive layer 119 b protrude from end portions of the conductivelayer 120 b. Further, the area of the conductive layer 119 a and thearea of the conductive layer 119 b are larger than the area of theconductive layer 120 a and the area of the conductive layer 120 b,respectively.

For the conductive layers 110 a, 120 a, and 120 b, for example, it ispreferable to use metal materials in order to lower the wiringresistance of the wirings.

The gate wiring in the driver circuit portion is formed using the stackof the conductive layer 107 a and the conductive layer 110 a. The sourcewiring which is electrically connected to the source electrode layer orthe drain electrode layer of the thin film transistor is formed usingthe stack of the conductive layer 119 a and the conductive layer 120 aor the stack of the conductive layer 119 b and the conductive layer 120b. In other words, the gate electrode layer of the thin film transistoris formed using part of the stack of the conductive layer 107 a and theconductive layer 110 a which are included in the gate wiring, and thesource electrode layer or the drain electrode layer is formed using partof the stack of the conductive layer 119 a and the conductive layer 120a which are included in the source wiring or part of the stack of theconductive layer 119 b and the conductive layer 120 b.

Note that in this specification, when it is explicitly described that “Xand Y are connected”, the case where X and Y are electrically connected,the case where X and Y are functionally connected, and the case where Xand Y are directly connected are included therein. Here, each of X and Yis an object (e.g., a device, an element, a circuit, a wiring, anelectrode, a terminal, a conductive film, or a layer). Therefore,another element may be interposed between elements having a connectionrelationship illustrated in drawings and texts, without limitation to apredetermined connection relation, for example, the connection relationillustrated in the drawings and the texts.

Note that terms such as “first”, “second”, and “third” are used fordistinguishing various elements, members, regions, layers, areas, andthe like from others. Therefore, the terms such as “first”, “second”,and “third” do not limit the order and the number of the elements,members, regions, layers, areas, and the like. Further, for example, theterm “first” can be replaced with the term “second”, “third”, or thelike.

In addition, as illustrated in FIGS. 1A to 1C, the thin film transistor130A provided in the driver circuit can include a second gate electrodelayer (also referred to as a back gate electrode layer) including aconductive layer 400 a and a conductive layer 401 a above the channelformation region. When the back gate electrode layer is electricallyconnected to the lower gate electrode layer and has the same potentialas the lower gate electrode layer, gate voltage can be applied fromupper and lower sides of the semiconductor layer which is providedbetween the lower gate electrode layer and the back gate electrodelayer. Further, when the lower gate electrode layer and the back gateelectrode layer have different potentials, for example, when thepotential of the back gate electrode layer is a fixed potential such asa ground potential (also referred to as GND) or 0 V, electricalcharacteristics of the TFT, for example, the threshold voltage or thelike can be controlled. In other words, when the stack of the conductivelayer 107 a and the conductive layer 110 a functions as a first gateelectrode layer and the stack of the conductive layer 400 a and theconductive layer 401 a functions as a second gate electrode layer, thethin film transistor 130A can be used as a thin film transistor havingfour terminals.

Further, the driver circuit portion illustrated in FIGS. 1A to 1Cincludes an insulating layer 123 between the conductive layer 400 a andthe semiconductor layer 113 a, the conductive layer 119 a, theconductive layer 119 b, the conductive layer 120 a, and the conductivelayer 120 b.

For example, the insulating layer 123 can be formed using a single layerof an insulating film or a stack of insulating films.

Further, an oxide insulating film can be provided between the insulatinglayer 123 and the semiconductor layer 113 a. With the provision of theoxide insulating film, the carrier concentration of the semiconductorlayer can be lowered.

Over the substrate 101 having an insulating surface, a thin filmtransistor 130B in a pixel illustrated in FIGS. 2A to 2C includes aconductive layer 107 e having a function as a gate electrode layer or agate wiring, a gate insulating layer, a semiconductor layer 113 eincluding a channel formation region, a conductive layer 119 h having afunction as a source electrode layer or a source wiring, and aconductive layer 119 e having a function as a drain electrode layer.

The conductive layer 107 e, the semiconductor layer 113 e, theconductive layer 119 e, and the conductive layer 119 h can be formedusing light-transmitting materials. Therefore, the entire thin filmtransistor 130B can be formed using light-transmitting materials.

Note that in this specification, a light-transmitting layer (film)refers to a layer (film) whose transmittance of visible light is 75 to100%. In the case where such a layer (film) has conductivity, it is alsoreferred to as a light-transmitting conductive layer (film). Inaddition, a conductive film having translucence with respect to visiblelight may be used for a metal oxide used for a gate electrode layer, asource electrode layer, a drain electrode layer, a pixel electrode, adifferent electrode, or a different wiring. Translucence with respect tovisible light refers to a transmittance of 50 to 75%.

For example, an oxide semiconductor can be used for the semiconductorlayer 113 a or the semiconductor layer 113 e. As for the oxidesemiconductor, in the case where heat treatment is performed in theatmosphere of an inert gas such as nitrogen or a rare gas (e.g., argonor helium) or under reduced pressure, the oxide semiconductor layer ischanged into an oxygen-deficient oxide semiconductor layer by the heattreatment so as to be a low-resistant oxide semiconductor layer, i.e.,an n-type (e.g., n⁻-type) oxide semiconductor layer. After that, byforming an oxide insulating film which is in contact with the oxidesemiconductor layer so that the oxide semiconductor layer is made to bein an oxygen-excess state, the oxide semiconductor layer can be changedinto a high-resistant oxide semiconductor layer, i.e., an i-type oxidesemiconductor layer. Thus, it is possible to manufacture a semiconductordevice including a highly reliable thin film transistor with favorableelectrical characteristics.

In dehydration or dehydrogenation, heat treatment is performed at atemperature which is higher than or equal to 350° C., preferably higherthan or equal to 400° C., and lower than the strain point of a substratein the atmosphere of an inert gas such as nitrogen or a rare gas (e.g.,argon or helium) or under reduced pressure, so that an impurity such asmoisture contained in the oxide semiconductor layer is reduced.

The heat treatment for dehydration or dehydrogenation is performed undera heat treatment condition that two peaks of water or at least one peakof water at around 300° C. is not detected even if TDS is performed atup to 450° C. on the dehydrated or dehydrogenated oxide semiconductorlayer. Therefore, even if TDS is performed at up to 450° C. on a thinfilm transistor including such a dehydrated or dehydrogenated oxidesemiconductor layer, at least the peak of water at around 300° C. is notdetected.

In addition, it is important not to mix water or hydrogen into the oxidesemiconductor layer without exposure of the oxide semiconductor layer tothe air with the use of a furnace used for dehydration ordehydrogenation when the temperature is lowered from a heatingtemperature T at which dehydration or dehydrogenation is performed onthe oxide semiconductor layer. When a thin film transistor is formedusing an oxide semiconductor layer obtained by changing an oxidesemiconductor layer into a low-resistant oxide semiconductor layer,i.e., an n-type (e.g., n⁻-type) oxide semiconductor layer by dehydrationor dehydrogenation and then by changing the low-resistant oxidesemiconductor layer into a high-resistant oxide semiconductor layer soas to be an i-type semiconductor layer, the threshold voltage of thethin film transistor can be positive voltage, so that a so-callednormally-off switching element can be realized. It is preferable for adisplay device that a channel be formed with positive threshold voltageand as close to 0 V as possible in a thin film transistor. Note that ifthe threshold voltage of the thin film transistor is negative, the thinfilm transistor tends to be normally on; in other words, current flowsbetween a source electrode layer and a drain electrode layer even whengate voltage is 0 V. In an active matrix display device, the electricalcharacteristics of a thin film transistor included in a circuit areimportant and influence the performance of the display device. Among theelectrical characteristics of the thin film transistor, the thresholdvoltage (V_(th)) is particularly important. When the threshold voltageis high or negative even when field-effect mobility is high, it isdifficult to control the circuit. In the case where a thin filmtransistor has high threshold voltage and a large absolute value of itsthreshold voltage, the thin film transistor cannot perform a switchingfunction as the TFT and might be a load when the TFT is driven at lowvoltage. In the case of an n-channel thin film transistor, it ispreferable that a channel be formed and drain current flows afterpositive voltage is applied as gate voltage. A thin film transistor inwhich a channel is not formed unless driving voltage is raised and athin film transistor in which a channel is formed and drain currentflows even when negative voltage is applied are unsuitable for a thinfilm transistor used in a circuit.

In addition, the gas atmosphere in which the temperature is lowered fromthe heating temperature T may be switched to a gas atmosphere which isdifferent from the gas atmosphere in which the temperature is raised tothe heating temperature T. For example, cooling is performed while thefurnace used for dehydration or dehydrogenation is filled with ahigh-purity oxygen gas or a high-purity N₂O gas without exposure of theoxide semiconductor layer to the air.

With the use of an oxide semiconductor film cooled slowly (or cooled) inan atmosphere which does not contain moisture (having a dew point of−40° C. or lower, preferably −60° C. or lower) after an impurity such asmoisture contained in the film is reduced by heat treatment fordehydration or dehydrogenation, the electrical characteristics of a thinfilm transistor are improved and high-performance thin film transistorswhich can be mass-produced are realized.

In this specification, heat treatment in the atmosphere of an inert gassuch as nitrogen or a rare gas (e.g., argon or helium) or under reducedpressure is referred to as heat treatment for dehydration ordehydrogenation. In this specification, for convenience, dehydration ordehydrogenation refers not only to elimination of H₂ but also toelimination of H, OH, or the like.

In the case where heat treatment is performed in the atmosphere of aninert gas such as nitrogen or a rare gas (e.g., argon or helium) orunder reduced pressure, the oxide semiconductor layer is changed into anoxygen-deficient oxide semiconductor layer by the heat treatment so asto be a low-resistant oxide semiconductor layer, i.e., an n-type (e.g.,n⁻-type) oxide semiconductor layer. After that, a region overlappingwith a source electrode layer is formed as a high-resistant sourceregion (also referred to as an HRS region) which is an oxygen-deficientregion, and a region overlapping with a drain electrode layer is formedas a high-resistant drain region (also referred to as an HRD region)which is an oxygen-deficient region. For example, in the thin filmtransistor illustrated in FIGS. 1A to 1C, a high-resistant source regioncan be formed in a region of the semiconductor layer 113 a, whichoverlaps with the conductive layer 119 a, and a high-resistant drainregion can be formed in a region of the semiconductor layer 113 a, whichoverlaps with the conductive layer 119 b. Further, in the thin filmtransistor illustrated in FIGS. 2A to 2C, a high-resistant source regioncan be formed in a region of the semiconductor layer 113 e, whichoverlaps with the conductive layer 119 e, and a high-resistant drainregion can be formed in a region of the semiconductor layer 113 e, whichoverlaps with the conductive layer 119 h.

The carrier concentration of the high-resistant source region or thehigh-resistant drain region is higher than or equal to 1×10¹⁷/cm³ and isat least higher than the carrier concentration of a channel formationregion (lower than 1×10¹⁷/cm³). Note that the carrier concentration inthis specification is carrier concentration obtained by Hall effectmeasurement at room temperature.

Further, a low-resistant source region (also referred to as an LRSregion) and a low-resistant drain region (also referred to as an LRDregion) may be formed between the oxide semiconductor layer and thedrain electrode layer formed using a metal material. The carrierconcentration of the low-resistant drain region is higher than thecarrier concentration of the high-resistant drain region (the HRDregion), for example, higher than or equal to 1×10²⁰/cm³ and lower thanor equal to 1×10²¹/cm³. In the semiconductor device of this embodiment,the conductive layer 119 a illustrated in FIG. 1B corresponds to alow-resistant source region, and the conductive layer 119 b illustratedin FIG. 1B corresponds to a low-resistant drain region.

Then, a channel formation region is formed by making at least part ofthe dehydrated or dehydrogenated oxide semiconductor layer be in anoxygen-excess state so as to be a higher-resistant oxide semiconductorlayer, i.e., an i-type oxide semiconductor layer. Note that as thetreatment for making part of the dehydrated or dehydrogenated oxidesemiconductor layer be in an oxygen-excess state, any of the followingmethods is employed: deposition of an oxide insulating film which is incontact with the dehydrated or dehydrogenated oxide semiconductor layerby sputtering; heat treatment after the deposition of the oxideinsulating film; heat treatment in an atmosphere containing oxygen afterthe deposition of the oxide insulating film; cooling treatment in anoxygen atmosphere after heat treatment in an inert gas atmosphere afterthe deposition of the oxide insulating film; and cooling treatment inultra-dry air (having a dew point of −40° C. or lower, preferably −60°C. or lower) after heat treatment in an inert gas atmosphere after thedeposition of the oxide insulating film.

Further, at least part of the dehydrated or dehydrogenated oxidesemiconductor layer (a portion overlapping with a gate electrode (alsoreferred to as a gate electrode layer)) can be selectively made to be inan oxygen-excess state so as to be a high-resistant oxide semiconductorlayer, i.e., an i-type oxide semiconductor layer. Thus, the channelformation region can be formed. For example, the channel formationregion can be formed in such a manner that a source electrode layer anda drain electrode layer formed using metal electrodes of Ti or the likeare formed on and in contact with the dehydrated or dehydrogenated oxidesemiconductor layer and exposure regions which do not overlap with atleast one of the source electrode layer and the drain electrode layerare selectively made to be in an oxygen-excess state. In the case wherethe exposure regions are selectively made to be in an oxygen-excessstate, a high-resistant source region overlapping with the sourceelectrode layer and a high-resistant drain region overlapping with thedrain electrode layer are formed, and the channel formation region isformed between the high-resistant source region and the high-resistantdrain region. That is, the channel formation region is formed betweenthe source electrode layer and the drain electrode layer in aself-aligning manner.

Thus, it is possible to manufacture a semiconductor device including ahighly reliable thin film transistor with favorable electricalcharacteristics.

Note that by forming the high-resistant drain region in the oxidesemiconductor layer overlapping with the drain electrode layer (and thesource electrode layer), reliability when a driver circuit is formed canbe improved. Specifically, by forming the high-resistant drain region, astructure can be employed in which conductivity can be varied stepwisefrom the drain electrode layer to the channel formation region via thehigh-resistant drain region. Therefore, in the case where operation isperformed with the drain electrode layer connected to a wiring forsupplying a high power supply potential VDD, the high-resistant drainregion serves as a buffer and a high electric field is not appliedlocally even if the high electric field is applied between the gateelectrode layer and the drain electrode layer, so that the withstandvoltage of the thin film transistor can be improved.

In addition, by forming the high-resistant drain region (or thehigh-resistant source region) in the oxide semiconductor layeroverlapping with the drain electrode layer (and the source electrodelayer), the amount of leakage current in the channel formation regionwhen the driver circuit is formed can be reduced. Specifically, byforming the high-resistant drain region (or the high-resistant sourceregion), the leakage current of the thin film transistor, which flowsbetween the drain electrode layer and the source electrode layer, flowssequentially through the drain electrode layer, the high-resistant drainregion on the drain electrode layer side, the channel formation region,the high-resistant source region on the source electrode layer side, andthe source electrode layer. In this case, in the channel formationregion, leakage current flowing from the low-resistant drain region onthe drain electrode layer side to the channel formation region can belocalized in the vicinity of an interface between the channel formationregion and a gate insulating layer which has high resistance when thethin film transistor is off. Thus, the amount of leakage current in aback channel portion (part of a surface of the channel formation region,which is apart from the gate electrode layer) can be reduced.

Further, the high-resistant source region overlapping with the sourceelectrode layer and the high-resistant drain region overlapping with thedrain electrode layer are formed so as to overlap with part of the gateelectrode layer, so that the intensity of an electric field in thevicinity of an end portion of the drain electrode layer can be reducedmore effectively.

The gate wiring which is electrically connected to the gate electrodelayer of the thin film transistor 130B in the pixel portion is formedusing the conductive layer 107 e. The source wiring which iselectrically connected to the source electrode layer or the drainelectrode layer of the thin film transistor 130B in the pixel portion isformed using the conductive layer 119 e or the conductive layer 119 h.In other words, the gate electrode layer of the thin film transistor130B is formed using part of the conductive layer 107 e used for thegate wiring, and the source electrode layer or the drain electrode layerof the thin film transistor 130B is formed using part of the conductivelayer 119 e or part of the conductive layer 119 h used for the sourcewiring.

Note that a wiring having a function as the gate electrode layer can beconsidered to be connected to a wiring having a function as the gatewiring (or at least one of layers of the wiring functioning as the gatewiring). Alternatively, at least one of the layers of the gate wiringcan be formed to have a larger area than the other layer of the gatewiring, and part of the region with the larger area can be considered tofunction as the gate electrode layer.

Alternatively, at least part of the gate wiring can be considered tofunction as the gate electrode layer or part of the gate electrodelayer. Alternatively, over a conductive layer which functions as thegate electrode layer or part of the gate electrode layer in the pixelportion and mainly functions as the gate electrode layer or part of thegate electrode layer of the thin film transistor, a conductive layerwhich mainly functions as the gate wiring or part of the gate wiring inthe driver circuit portion can be considered to be provided.

A wiring which has a function as the source wiring and includes thesource electrode layer of the thin film transistor in the pixel portioncan be considered to be connected to a wiring which has a function asthe source wiring and includes the source electrode layer of the thinfilm transistor in the driver circuit portion (or at least one of layersof the wiring which has a function as the source wiring and includes thesource electrode layer of the thin film transistor in the driver circuitportion). That is, part of the source wiring in the driver circuitportion can be considered to function as the source electrode layer inthe driver circuit portion or part of the source electrode layer in thepixel portion. Alternatively, over a conductive layer which mainlyfunctions as the source electrode layer or part of the source electrodelayer in the pixel portion, a conductive layer which mainly functions asthe source wiring or part of the source wiring in the driver circuitportion can be considered to be provided.

In addition, the thin film transistor 130B provided in the pixel portioncan include a second gate electrode layer (also referred to as a backgate electrode layer) including a conductive layer 400 e above thechannel formation region. When the back gate electrode layer iselectrically connected to the lower gate electrode layer and has thesame potential as the lower gate electrode layer, gate voltage can beapplied from upper and lower sides of the semiconductor layer which isprovided between the lower gate electrode layer and the back gateelectrode layer. Further, when the lower gate electrode layer and theback gate electrode layer have different potentials, for example, whenthe potential of the back gate electrode layer is a fixed potential suchas GND or 0 V, electrical characteristics of the TFT, for example, thethreshold voltage or the like can be controlled.

In addition, the pixel portion illustrated in FIGS. 2A to 2C includes astorage capacitor which is formed using a conductive layer 107 gfunctioning as a lower electrode, the insulating film 111 which has afunction as a gate insulating layer and serves as a dielectric, and aconductive layer 119 g functioning as an upper electrode. A storagecapacitor line is formed using the conductive layer 107 g and theconductive layer 119 g. Further, an insulating layer 123 is providedbetween the conductive layer 400 e and the semiconductor layer 113 e andthe conductive layers 119 h and 119 e. Since the insulating layer 122 issimilar to the insulating layer 123 in FIG. 1B, description thereof isomitted.

Since the conductive layers 107 g and 119 g are formed usinglight-transmitting materials, at least part of a region of one of theconductive layers 107 g and 119 g can have a function as a capacitorwiring or part of a capacitor wiring, and part of a region of the otherof the conductive layers 107 g and 119 g can function as an electrode ofa capacitor or part of an electrode of a capacitor. Note that althoughFIGS. 2A to 2C illustrate the case where a capacitor is provided in thepixel portion, this embodiment is not limited to this. A capacitor canbe provided in the driver circuit portion. For example, in the casewhere at least part of the region of one of the conductive layers 107 gand 119 g is a region where a light-transmitting conductive layeroverlaps with a conductive layer whose resistance value is lower thanthat of the light-transmitting layer and the conductive layer whoseresistance value is lower than that of the light-transmitting layer is alight-blocking conductive layer, at least part of the region of one ofthe conductive layers 107 g and 119 g preferably functions as acapacitor wiring or part of a capacitor wiring in the driver circuitportion. Further, in a region where a light-blocking conductive layer isnot provided and a light-transmitting conductive layer is provided, partof the region of the other of the conductive layers 107 g and 119 gpreferably functions as an electrode of a capacitor or part of anelectrode of a capacitor in the pixel portion.

In addition, in the semiconductor device of this embodiment, a wiringhaving a function as an electrode of a capacitor can be considered to beconnected to a wiring functioning as a capacitor wiring (or at least oneof layers of the wiring functioning as the capacitor wiring).Alternatively, at least one of the layers of the capacitor wiring can beformed to have a larger area than the other layer of the capacitorwiring, and part of the region with the larger area can be considered tofunction as the electrode of the capacitor. Further, thelight-transmitting conductive layer can be considered to be formed tohave a larger area than the light-blocking conductive layer and part ofthe region with the larger area of the conductive layer can beconsidered to function as the electrode of the capacitor. Furthermore,at least part of the capacitor wiring in the pixel portion can beconsidered to function as the electrode of the capacitor or part of theelectrode of the capacitor. Alternatively, at least one of the layers ofthe capacitor wiring can be considered to function as the electrode ofthe capacitor. Alternatively, part of the light-transmitting conductivelayer can be considered to function as the electrode of the capacitor.Alternatively, over a conductive layer which mainly functions as theelectrode of the capacitor or part of the electrode of the capacitor inthe pixel portion, a conductive layer which mainly functions as thecapacitor wiring or part of the capacitor wiring in the driver circuitportion can be considered to be provided.

In addition, part of a region in the light-blocking conductive layer orthe light-transmitting conductive layer (mainly, a region in thelight-blocking conductive layer) can function as a capacitor wiring ledfrom an FPC or part of the capacitor wiring in the driver circuitportion, and another part of the region (a region including only thelight-transmitting conductive layer) can function as the electrode ofthe capacitor in the pixel portion or part of the electrode of thecapacitor. It is preferable that a region where the light-blockingconductive layer and the light-transmitting conductive layer overlapwith each other function as the capacitor wiring led from the FPC or thepart of the capacitor wiring, because the region has high conductivity(has a low resistance value) and includes the light-blocking conductivelayer in some cases. Alternatively, it is preferable that thelight-transmitting conductive layer in the region where thelight-blocking conductive layer is not provided function as theelectrode of the capacitor in the pixel portion or the part of theelectrode of the capacitor.

Note that in the case where the thin film transistor is formed over thegate wiring, the size of the thin film transistor depends on the widthof the gate wiring of the thin film transistor. However, in thisembodiment, since the thin film transistor is formed in a pixel, thesize of the thin film transistor can be made large. Note that thisembodiment is not limited to this. For example, as illustrated in FIGS.32A and 32B, a thin film transistor whose width is larger than the widthof a gate wiring can be formed. By making a thin film transistor larger,its current supply capability can be sufficiently increased, and thewriting time of a signal to the pixel can be shortened. Therefore, ahigh-definition display device can be provided.

A storage capacitor portion includes a light-transmitting conductivelayer which functions as a lower electrode, with an insulating filmserving as a gate insulating film used as a dielectric. Therefore, byforming the storage capacitor portion with the use of thelight-transmitting conductive layer as described above, the apertureratio can be improved. In addition, by forming the storage capacitorportion with the use of the light-transmitting conductive layer, thestorage capacitor portion can be made large, so that a potential of apixel electrode can be easily held even if the thin film transistor isturned off. Further, a feedthrough potential can be lowered.

As described above, in the semiconductor device illustrated in FIGS. 1Ato 1C and FIGS. 2A to 2C, the driver circuit portion and the pixelportion each including a thin film transistor are formed over the samesubstrate. The gate electrode layer and the source electrode layer ofthe thin film transistor in the pixel portion are formed usinglight-transmitting conductive layers. The semiconductor layer of thethin film transistor in the pixel portion is formed using alight-transmitting semiconductor material. The gate electrode layer andthe source electrode layer of the thin film transistor in the drivercircuit portion are formed using conductive layers whose resistancevalues are lower than those of the light-transmitting conductive layers.With this structure, the aperture ratio in the pixel portion can beimproved; higher resolution can be realized; distortion in the waveformof a signal is suppressed by the decrease in wiring resistance in thedriver circuit portion; power consumption can be reduced; and operationspeed can be improved. Further, the larger the semiconductor devicebecomes, the more wiring resistance influences the semiconductor device.Therefore, the structure of the semiconductor device in this embodimentis also preferable when a semiconductor device is made larger.

Alternatively, in the semiconductor device illustrated in FIGS. 1A to 1Cand FIGS. 2A to 2C, an electrode and a wiring of the storage capacitorin the pixel portion can be formed using light-transmitting conductivelayers. With this structure, the aperture ratio can be improved, and thedecrease in the aperture ratio can be suppressed even in the case wherethe area of the storage capacitor is made large.

Alternatively, in the semiconductor device illustrated in FIGS. 1A to 1Cand FIGS. 2A to 2C, a lead wiring such as a power supply line or asignal line, the gate wiring, and the source wiring in the pixel portioncan be formed using light-transmitting conductive layers, and a leadwiring such as a power supply line or a signal line, the gate wiring,and the source wiring in the driver circuit portion can be formed usingconductive layers whose resistance values are lower than those of thelight-transmitting conductive layers. With this structure, distortion inthe waveform of a signal is suppressed, power consumption can bereduced, and operation speed can be improved.

Alternatively, the semiconductor device illustrated in FIGS. 1A to 1Cand FIGS. 2A to 2C can include a light-transmitting conductive layeroverlapping with the channel formation region of the thin filmtransistor in the pixel portion, and a conductive layer which is formedusing a conductive material whose resistance value is lower than that ofa light-transmitting conductive material and which overlaps with thechannel formation region of the thin film transistor in the drivercircuit portion. The conductive layers (the conductive layersoverlapping with the channel formation regions) which are provided inthe pixel portion and the driver circuit portion can function as secondelectrodes (back gate electrode layers) of the thin film transistorsprovided in the pixel portion and the driver circuit portion. Theconductive layers are not necessarily provided; however, when the backgate electrode layers are provided, the threshold voltage of the thinfilm transistors can be controlled and the reliability of the thin filmtransistors can be improved.

Next, an example of a method for manufacturing the semiconductor deviceof this embodiment is described with reference to FIGS. 3A to 3D, FIGS.4A to 4D, FIGS. 5A to 5D, FIGS. 6A to 6D, FIGS. 7A to 7C, FIGS. 8A to8C, FIGS. 9A to 9D, FIGS. 10A to 10D, FIGS. 11A to 11D, and FIGS. 12A to12E. FIGS. 3A to 3D, FIGS. 5A to 5D, FIGS. 7A to 7C, FIGS. 9A to 9D, andFIGS. 10A to 10D each illustrate a cross section taken along line A-B inFIG. 1A. FIGS. 4A to 4D, FIGS. 6A to 6D, FIGS. 8A to 8C, FIGS. 11A to11D, and FIGS. 12A to 12E each illustrate a cross section taken alongline E-F in FIG. 2A. FIGS. 3A to 3D, FIGS. 5A to 5D, FIGS. 7A to 7C,FIGS. 9A to 9D, and FIGS. 10A to 10D illustrate a source wiring portion301, a thin film transistor portion 302, and a gate wiring portion 303.FIGS. 4A to 4D, FIGS. 6A to 6D, FIGS. 8A to 8C, FIGS. 11A to 11D, andFIGS. 12A to 12E illustrate a source wiring portion 331, a thin filmtransistor portion 332, a gate wiring portion 333, and a storagecapacitor portion 334. Note that in the manufacturing method illustratedin FIGS. 3A to 3D, FIGS. 4A to 4D, FIGS. 5A to 5D, FIGS. 6A to 6D, FIGS.7A to 7C, FIGS. 8A to 8C, FIGS. 9A to 9D, FIGS. 10A to 10D, FIGS. 11A to11D, and FIGS. 12A to 12E, a multi-tone mask is used, for example;however, this embodiment is not limited to this.

First, as illustrated in FIG. 3A and FIG. 4A, a conductive film 102 anda conductive film 103 are stacked over the substrate 101 by sputtering.These steps can be performed successively and sequential sputtering canbe performed using a multi-chamber. By successively forming theconductive film 102 and the conductive film 103, throughput is improvedand contamination by an impurity or dust can be suppressed.

The substrate 101 is preferably formed using a material having hightransmittance. For example, a glass substrate, a plastic substrate, anacrylic substrate, a ceramic substrate, or the like can be used.

It is preferable that the transmittance of the conductive film 102 besufficiently high. Further, the transmittance of the conductive film 102is preferably higher than the transmittance of the conductive film 103.

The conductive film 102 can be formed using a conductive material havinga light-transmitting property with respect to visible light, forexample, an In—Sn—Zn—O-based metal oxide, an In—Al—Zn—O-based metaloxide, a Sn—Ga—Zn—O-based metal oxide, an A—Ga—Zn—O-based metal oxide, aSn—Al—Zn—O-based metal oxide, an In—Zn—O-based metal oxide, aSn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, an In—O-basedmetal oxide, a Sn—O-based metal oxide, or a Zn—O-based metal oxide canbe used. The metal oxide can be formed by, for example, sputtering,vacuum evaporation (e.g., electron beam deposition), arc discharge ionplating, or a spray method. In addition, in the case where sputtering isused, deposition may be performed using a target containing SiO₂ at 2 to10 wt %, and SiO_(x) (x>0), which inhibits crystallization, may becontained in the light-transmitting conductive film. Thus,crystallization of the metal oxide is suppressed when heat treatment fordehydration or dehydrogenation is performed in a later step.Alternatively, the conductive film 102 may be formed by stacking aplurality of films including any of the above materials. In the case ofthe layered structure, it is preferable that the transmittance of eachof the plurality of films be sufficiently high.

It is preferable that the resistance value of the conductive film 103 besufficiently low and the conductivity of the conductive film 103 besufficiently high. In addition, the resistance value of the conductivefilm 102 is preferably lower than the resistance value of the conductivefilm 103. Since the conductive film 102 functions as a conductive layer,the resistance value of the conductive film 102 is preferably lower thanthe resistance value of an insulating layer.

The conductive film 103 can be formed to have a single-layer structureor a layered structure with the use of a metal material such asmolybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper,neodymium, or scandium, or an alloy material containing the abovematerial as a main component by sputtering or vacuum evaporation. Inaddition, in the case where the conductive film 103 is formed to have alayered structure, a light-transmitting conductive film may be includedas any of plurality of films.

Note that in the case where the conductive film 103 is formed over theconductive film 102, both the films react with each other in some cases.For example, in the case where a top surface (a surface which is incontact with the conductive film 103) of the conductive film 102 isformed using ITO and a bottom surface (a surface which is in contactwith the conductive film 102) of the conductive film 103 is formed usingaluminum, chemical reaction occurs therebetween in some cases.Therefore, in order to avoid such chemical reaction, a high-meltingpoint material is preferably used for the bottom surface (the surfacewhich is in contact with the conductive film 102) of the conductive film103. For example, as the high-melting point material, molybdenum (Mo),titanium (Ti), tungsten (W), neodymium (Nd), or the like can be used. Itis preferable to form the conductive film 103 as a multi-layer film withthe use of a material having a low resistance value over the film formedusing the high-melting point material. As the material having a lowresistance value, aluminum (Al), copper (Cu), silver (Ag), or the likecan be used. For example, in the case where the conductive film 103 isformed to have a layered structure, a stack of molybdenum (Mo) as afirst layer, aluminum (Al) as a second layer, and molybdenum (Mo) as athird layer, or a stack of molybdenum (Mo) as a first layer, aluminum(Al) containing a small amount of neodymium (Nd) as a second layer, andmolybdenum (Mo) as a third layer can be used.

Although not illustrated, silicon oxide, silicon nitride, siliconoxynitride, or the like can be formed between the substrate 101 and theconductive film 102 as a base film. By forming the base film between thesubstrate 101 and the light-transmitting conductive film, diffusion ofmobile ions, impurities, or the like from the substrate 101 into anelement can be suppressed, so that deterioration in the characteristicsof the element can be prevented.

Next, as illustrated in FIG. 3B and FIG. 4B, over the conductive film103, resist masks 106 a and 106 b having large thickness are formed inthe driver circuit portion and resist masks 106 e, 106 f, and 106 ghaving smaller thickness than the resist masks 106 a and 106 b areformed in the pixel portion. The resist masks 106 a, 106 b, 106 e, 106f, and 106 g can be formed using a multi-tone mask, for example. Withthe use of a multi-tone mask, a resist mask having regions withdifferent thicknesses can be formed. With the use of the multi-tonemask, the number of photomasks used and the number of manufacturingsteps are reduced. In this embodiment, the multi-tone mask can be usedin a step of forming the patterns of the conductive film 102 and theconductive film 103 and a step of forming the light-transmittingconductive layer which functions as the gate electrode layer.

A multi-tone mask is a mask capable of light exposure with multi-levellight intensity, typically, with three levels of light intensity so thatan exposed region, a semi-exposed region, and an unexposed region areformed. With the use of the multi-tone mask, a resist mask with pluralthicknesses (typically two kinds of thicknesses) can be formed byone-time exposure and development process. Therefore, with the use ofthe multi-tone mask, the number of photomasks can be reduced.

FIGS. 16A-1 and 16B-1 illustrate cross sections of typical multi-tonemasks. FIG. 16A-1 illustrates a gray-tone mask 180, and FIG. 16B-1illustrates a half-tone mask 185.

The gray-tone mask 180 illustrated in FIG. 16A-1 includes alight-blocking portion 182 formed using a light-blocking layer on alight-transmitting substrate 181, and a diffraction grating portion 183provided with a pattern of the light-blocking layer.

The diffraction grating portion 183 has slits, dots, meshes, or the likeprovided at intervals which are less than or equal to the resolutionlimit of light used for exposure, so that the amount of light to betransmitted is controlled. Note that the slits, dots, or meshes providedat the diffraction grating portion 183 may be provided periodically ornon-periodically.

For the light-transmitting substrate 181, quartz or the like can beused. The light-blocking layer included in the light-blocking portion182 and the diffraction grating portion 183 may be formed using a metalfilm, and is preferably formed using chromium, chromium oxide, or thelike.

In the case where the gray-tone mask 180 is irradiated with light forexposure, as illustrated in FIG. 16A-2, transmittance in a regionoverlapping with the light-blocking portion 182 is 0% and transmittancein a region where neither the light-blocking portion 182 nor thediffraction grating portion 183 is provided is 100%. Further,transmittance at the diffraction grating portion 183 is approximately inthe range of 10 to 70%, which can be adjusted by the interval of slits,dots, or meshes of the diffraction grating, or the like.

The half-tone mask 185 illustrated in FIG. 16B-1 includes asemi-light-transmitting portion 187 formed using asemi-light-transmitting layer on a light-transmitting substrate 186, anda light-blocking portion 188 formed using a light-blocking layer.

The semi-light-transmitting portion 187 can be formed using a layer ofMoSiN, MoSi, MoSiO, MoSiON, CrSi, or the like. The light-blockingportion 188 may be formed using a metal film which is similar to that ofthe light-blocking layer of the gray-tone mask, and is preferably formedusing chromium, chromium oxide, or the like.

In the case where the half-tone mask 185 is irradiated with light forexposure, as illustrated in FIG. 16B-2, transmittance in a regionoverlapping with the light-blocking portion 188 is 0%, and transmittancein a region where neither the light-blocking portion 188 nor thesemi-light-transmitting portion 187 is provided is 100%. Further,transmittance at the semi-light-transmitting portion 187 isapproximately in the range of 10 to 70%, which can be adjusted by thekind, thickness, or the like of a material to be used.

By performing exposure with the use of the multi-tone mask anddevelopment, a resist mask having regions with different thicknesses canbe formed. In addition, a resist mask with different thicknesses can beformed.

Next, as illustrated in FIG. 3C and FIG. 4C, etching is performed usingthe resist masks 106 a, 106 b, 106 e, 106 f, and 106 g. With theetching, the conductive film 102 and the conductive film 103 areselectively removed, so that conductive layers 107 a, 108 a, 107 b, 108b, 107 e, 108 e, 107 f, 108 f, 107 g, and 108 g can be formed.

Then, as illustrated in FIG. 3D and FIG. 4D, ashing is performed on theresist masks 106 a, 106 b, 106 e, 106 f, and 106 g. For example, ashingor the like in which oxygen plasma is used may be performed. When theresist masks 106 a and 106 b are reduced (downsized) by the ashing,resist masks 109 a and 109 b are formed and some of the conductivelayers 108 a and 108 b are exposed. Further, with this ashing treatment,the resist masks 106 e, 106 f, and 106 g in the pixel portion, whichhave small thickness, are removed, and the conductive layers 108 e, 108f, and 108 g are exposed. In this manner, with the use of the resistmask formed using the multi-tone mask, a resist mask is not additionallyused, so that steps can be simplified.

Next, as illustrated in FIG. 5A and FIG. 6A, etching is performed usingthe resist masks 109 a and 109 b. Thus, part of the conductive layer 108a is removed; a conductive layer 110 a is formed; a conductive layer 110b which is obtained by removal of the part of the conductive layer 108 bis formed; and the conductive layers 108 e, 108 f, and 108 g areremoved. After that, the resist masks 109 a and 109 b are removed. Byremoval of the part of the conductive layer 108 a, part of theconductive layer 107 a is exposed. By removal of the part of theconductive layer 108 b, part of the conductive layer 107 b is exposed.By removal of the conductive layer 108 e, the conductive layer 107 e isexposed. By removal of the conductive layer 108 f, the conductive layer107 f is exposed. By removal of the conductive layer 108 g, theconductive layer 107 g is exposed.

Note that as illustrated in FIG. 5A, with etching in which the resistmasks 109 a and 109 b which are obtained by reduction (downsizing) ofthe resist masks 106 a and 106 b are used, peripheral portions of theconductive layers 108 a and 108 b (regions in the conductive layers 108a and 108 b, which are exposed from the resist masks 109 a and 109 b)are etched concurrently. In other words, the end portions of theconductive layer 107 a protrude from end portions of the conductivelayer 108 a (110 a), and the end portions of the conductive layer 107 bprotrude from end portions of the conductive layer 108 b (110 b).Further, the area of the conductive layer 107 a and the area of theconductive layer 107 b are larger than the area of the conductive layer110 a and the area of the conductive layer 110 b, respectively.Furthermore, the conductive layers 110 a and 110 b and the conductivelayers 107 a and 107 b include a region where the conductive layer 110 aand the conductive layer 107 a overlap with each other, a region wherethe conductive layer 110 b and the conductive layer 107 b overlap witheach other, regions where the conductive layer 110 a and the conductivelayer 107 a do not overlap with each other, and regions where theconductive layer 11013 and the conductive layer 107 b do not overlapwith each other are provided.

When the light-blocking conductive layer is removed, part of thelight-transmitting conductive layer (for example, a surface portionwhich is in contact with the light-blocking conductive layer) is alsoremoved in some cases. The selectivity of the light-blocking conductivelayer to the light-transmitting conductive layer in etching determineshow much the light-transmitting conductive layer is removed. Therefore,for example, the thickness of the light-transmitting conductive layer ina region covered with the light-blocking conductive layer is oftenlarger than the thickness of the light-transmitting conductive layer ina region which is not covered with the light-blocking conductive layer.

In the case where only the light-blocking conductive layer is removed bywet etching while the light-transmitting conductive layer is left, anetching solution with high selectivity of the light-blocking conductivelayer to the light-transmitting conductive layer is used. In the casewhere a stack of molybdenum (Mo) as a first layer, aluminum (Al) as asecond layer, and molybdenum (Mo) as a third layer, or a stack ofmolybdenum (Mo) as a first layer, aluminum (Al) containing a smallamount of neodymium (Nd) as a second layer, and molybdenum (Mo) as athird layer is used as the light-blocking conductive layer, for example,a mixed acid of phosphoric acid, nitric acid, acetic acid, and water canbe used. With the use of this mixed acid, a forward tapered shape whichis uniform and favorable can be obtained. In this manner, in addition toimprovement in coverage due to a tapered shape, high throughput can beobtained while the wet etching is a simple process in which etching byan etchant, a rinse by pure water, and drying are performed. Thus, thewet etching is suitable for etching of the light-blocking conductivelayer.

Next, as illustrated in FIG. 5B and FIG. 6B, the insulating film 111which covers the conductive layers 107 a, 107 b, 107 e, 107 f, and 107 gand the conductive layers 110 a and 110 b and functions as a gateinsulating layer is formed.

The insulating film 111 may be formed to have a single-layer structureor a layered structure including a plurality of films. In the case ofthe layered structure including a plurality of films, it is preferablethat all the films have sufficiently high transmittance. In particular,in the pixel portion, it is preferable that all the films havesufficiently high transmittance.

The insulating film 111 which covers the light-transmitting conductivelayer and the light-blocking conductive layer is formed to a thicknessof about 50 to 500 nm. The insulating film 111 is formed to have asingle-layer structure of a film containing an oxide of silicon or anitride of silicon, or a layered structure thereof, by sputtering or avariety of CVD such as plasma-enhanced CVD. Specifically, the insulatingfilm 111 is formed by using a single layer of a film containing siliconoxide, a film containing silicon oxynitride, or a film containingsilicon nitride oxide, or by appropriately stacking these films.

The insulating film 111 is preferably formed using a light-transmittingmaterial or a material having high transmittance. Specifically, theinsulating film 111 is preferably formed using a material having highertransmittance than the conductive layers 107 a, 107 b, 107 e, 107 f, and107 g. Therefore, the transmittance of the insulating film 111 ispreferably higher than or equal to the transmittance of the conductivelayers 107 a, 107 b, 107 e, 107 f, and 107 g. This is because theinsulating film 111 is formed to have a large area in some cases andhigher transmittance is preferable in order to increase light useefficiency. In particular, in the pixel portion, it is preferable thatthe insulating film 111 and the conductive layers 107 e, 107 f, and 107g be formed using light-transmitting materials.

Next, a semiconductor film 112 is formed over the insulating film 111.

The semiconductor film 112 may be formed to have a single-layerstructure or a layered structure including a plurality of films. In thecase of the layered structure including a plurality of films, it ispreferable that all the films have sufficiently high transmittance.Similarly, especially in the pixel portion, it is preferable that allthe films have sufficiently high transmittance. The semiconductor film112 is preferably formed using a light-transmitting material or amaterial with high transmittance. The semiconductor film 112 can beformed using an oxide semiconductor, for example. For the oxidesemiconductor, any of the following oxide semiconductor films is used:an In—Ga—Zn—O-based non-single-crystal film; an In—Sn—Zn—O-based oxidesemiconductor film; an In—Al—Zn—O-based oxide semiconductor film; aSn—Ga—Zn—O-based oxide semiconductor film; an A—Ga—Zn—O-based oxidesemiconductor film; a Sn—Al—Zn—O-based oxide semiconductor film; anIn—Zn—O-based oxide semiconductor film; a Sn—Zn—O-based oxidesemiconductor film; an Al—Zn—O-based oxide semiconductor film; anIn—O-based oxide semiconductor film; a Sn—O-based oxide semiconductorfilm; and a Zn—O-based oxide semiconductor film. In this embodiment, thesemiconductor film 112 is formed by sputtering with the use of anIn—Ga—Zn—O-based oxide semiconductor target. Alternatively, the oxidesemiconductor film can be formed by sputtering in a rare gas (typicallyargon) atmosphere, an oxygen atmosphere, or an atmosphere including arare gas (typically argon) and oxygen. In addition, in the case wheresputtering is used, deposition is performed using a target containingSiO₂ at 2 to 10 wt %, and SiO_(x) (x>0), which inhibits crystallization,is contained in the oxide semiconductor film. Thus, crystallization canbe suppressed.

Note that before the semiconductor film 112 is formed by sputtering,dust on a surface of the insulating film 111 is preferably removed byreverse sputtering in which an argon gas is introduced and plasma isgenerated. The reverse sputtering refers to a method in which, withoutapplication of voltage to a target side, an RF power source is used forapplication of voltage to a substrate side in an argon atmosphere andplasma is generated in the vicinity of the substrate so that a substratesurface is modified. Note that nitrogen, helium, oxygen, or the like maybe used instead of the argon atmosphere.

Heat treatment for reducing an impurity such as moisture (heat treatmentfor dehydration or dehydrogenation) can be performed on the oxidesemiconductor film. The heat treatment leads to improvement inelectrical characteristics of the thin film transistor and improvementin reliability. For example, the heat treatment for dehydration ordehydrogenation is preferably performed at higher than or equal to 350°C. and lower than the strain point of the substrate, preferably higherthan or equal to 400° C. and lower than the strain point of thesubstrate. Here, after the substrate is put in an electric furnace whichis a kind of heat treatment apparatus and heat treatment is performed onthe oxide semiconductor film in a nitrogen atmosphere, it is preferablethat water or hydrogen be prevented from being mixed into the oxidesemiconductor film by preventing the substrate from being exposed to theair. Further, the same furnace is used from the heating temperature T atwhich the oxide semiconductor film is subjected to dehydration ordehydrogenation to a temperature low enough to prevent water fromentering again; specifically, slow cooling is performed in a nitrogenatmosphere until the temperature drops by 100° C. or more from theheating temperature T. Furthermore, without limitation to a nitrogenatmosphere, dehydration or dehydrogenation can be performed in a raregas atmosphere (e.g., helium, neon, or argon) or under reduced pressure.

Note that in the heat treatment, it is preferable that water, hydrogen,and the like be not included in nitrogen or a rare gas such as helium,neon, or argon. For example, the purity of nitrogen or a rare gas suchas helium, neon, or argon, which is introduced into the heat treatmentapparatus, is preferably 6N (99.9999%) or more, more preferably 7N(99.99999%) or more (i.e., impurity concentration is preferably 1 ppm orlower, more preferably 0.1 ppm or lower).

In addition, the transmittance of the conductive layers 107 a, 107 b,107 e, 107 f, and 107 g is preferably higher than or equal to thetransmittance of the semiconductor film 112. This is because theconductive layers 107 a, 107 b, 107 e, 107 f, and 107 g are used inlarge areas in some cases and the films having larger areas preferablyhave higher transmittance in order to improve light use efficiency andto reduce power consumption with higher aperture ratio. This is alsobecause the conductive layers 107 a, 107 b, 107 e, 107 f, and 107 g areused in a gate wiring portion, a source wiring portion, a thin filmtransistor portion, and a storage capacitor portion.

Further, the transmittance of the insulating film 111 is preferablyhigher than the transmittance of the semiconductor film 112. This isbecause the insulating film 111 is used in a larger area as compared tothe semiconductor film 112 in some cases and the film having a largerarea preferably has higher transmittance in order to improve light useefficiency.

Next, a resist mask (not illustrated) is formed over the semiconductorfilm 112. Then, etching is performed using the resist mask so thatsemiconductor layers 113 a and 113 e (also referred to as island-shapedsemiconductor layers) which are processed into desired shapes areformed, as illustrated in FIG. 5C and FIG. 6C. For the etching,hydrofluoric acid diluted to 0.05%, hydrochloric acid, or the like canbe used.

The semiconductor layers 113 a and 113 e can function as semiconductorlayers (active layers) of the thin film transistors or some of thesemiconductor layers (active layers) of the thin film transistors.Alternatively, the semiconductor layers 113 a and 113 e can function ascapacitors or some of the capacitors. Alternatively, the semiconductorlayers 113 a and 113 e can function as films for reducing parasiticcapacitance at the intersection portion of wirings.

Next, as illustrated in FIG. 5D and FIG. 6D, a conductive film 114 and aconductive film 115 are stacked by sputtering so as to cover thesemiconductor layer 113 a, the semiconductor layer 113 e, and theinsulating film 111. These steps can be performed successively andsequential sputtering can be performed using a multi-chamber. Bysuccessively forming the conductive film 114 and the conductive film115, throughput is improved and contamination by an impurity or dust canbe suppressed.

It is preferable that the transmittance of the conductive film 114 besufficiently high. Further, the transmittance of the conductive film 114is preferably higher than the transmittance of the conductive film 115.

The conductive film 114 can be formed to have a single-layer structureor a layered structure including one or a plurality of materials whichcan be used for the conductive film 102 illustrated in FIGS. 3A and 3Band FIGS. 4A and 4B.

The conductive film 114 is preferably formed using a material which issubstantially the same as the material of the conductive film 102.Substantially the same material is a material whose element of a maincomponent is the same. In terms of impurities, the kind and theconcentration of elements contained are different from each other insome cases. When the conductive film 114 is formed using the materialwhich is substantially the same as the material of the conductive film102 by sputtering or evaporation in this manner, there is an advantagethat the material can be shared between the conductive films 114 and102. When the material can be shared, the same manufacturing apparatuscan be used.

The resistance value of the conductive film 114 is preferably higherthan the resistance value of the conductive film 115.

The conductive film 115 can be formed to have a single-layer structureor a layered structure including one or a plurality of materials whichcan be used for the conductive film 103 illustrated in FIGS. 3A and 3Band FIGS. 4A and 4B.

Further, the conductive film 115 is preferably formed using a materialwhich is different from that used for the conductive film 103.Alternatively, the conductive film 115 is preferably formed to have alayered structure which is different from that of the light-blockingconductive film.

Note that in the case where the conductive film 115 is formed over theconductive film 114, both the films react with each other in some cases.For example, in the case where a top surface (a surface which is incontact with the conductive film 115) of the conductive film 114 isformed using ITO and a bottom surface (a surface which is in contactwith the conductive film 114) of the conductive film 115 is formed usingaluminum, chemical reaction occurs therebetween in some cases.Therefore, in order to avoid such chemical reaction, a high-meltingpoint material is preferably used for the bottom surface (the surfacewhich is in contact with the conductive film 114) of the conductive film115. For example, as the high-melting point material, molybdenum (Mo),titanium (Ti), tungsten (W), neodymium (Nd), or the like can be used. Itis preferable to form the conductive film 115 as a multi-layer film withthe use of a material having a low resistance value over the film formedusing the high-melting point material. As the material having a lowresistance value, aluminum (Al), copper (Cu), silver (Ag), or the likecan be used. Such a material has a light-blocking property andreflectivity.

Next, as illustrated in FIG. 7A and FIG. 8A, resist masks 118 a, 118 b,118 e, 118 g, and 118 h are formed over the conductive film 115. Theresist masks 118 a, 118 b, 118 e, 118 g, and 118 h are resist maskshaving regions with different thicknesses, which are obtained using amulti-tone mask. The thickness of the resist masks 118 a and 118 bprovided in the driver circuit portion is larger than the thickness ofthe resist masks 118 e, 118 g, and 118 h provided in the pixel portion.

Next, as illustrated in FIG. 7B and FIG. 8B, the conductive film 114 andthe conductive film 115 are etched using the resist masks 118 a, 118 b,118 e, 118 g, and 118 h. With the etching, the conductive layers 119 a,120 a, 119 b, 120 b, 119 e, 120 e, 119 g, 120 g, 119 h, and 120 h can beformed. Further, some of channel formation regions in the semiconductorlayers 113 a and 113 e can be etched.

Then, as illustrated in FIG. 7C and FIG. 8C, ashing is performed on theresist masks 118 a, 118 b, 118 e, 118 g, and 118 h. For example, ashingor the like in which oxygen plasma is used may be performed. When theresist masks 118 a and 118 b are reduced (downsized) by the ashing,resist masks 121 a and 121 b are formed and some of the conductivelayers 120 a and 120 b are exposed. Further, with this ashing treatment,the resist masks 118 e, 118 g, and 118 h in the pixel portion, whichhave small thickness, are removed, and the conductive layers 120 e, 120g, and 120 h are exposed. In this manner, with the use of the resistmask formed using the multi-tone mask, a resist mask is not additionallyused, so that steps can be simplified.

Next, as illustrated in FIG. 9A and FIG. 11A, the conductive layers 120a, 120 b, 120 e, 120 g, and 120 h are etched using the resist masks 121a and 121 b. Thus, conductive layers 104 a and 104 b which are obtainedby removal of some of the conductive layers 120 a and 120 b are formed,and some of the conductive layers 119 a and 119 b are exposed. Note thatthe end portions of the conductive layer 119 a protrude from endportions of the conductive layer 104 a, and the end portions of theconductive layer 119 b protrude from end portions of the conductivelayer 104 b. By removal of the conductive layers 120 e, 120 g, and 120h, the conductive layers 119 e, 119 g, and 119 h are exposed. After theetching, the resist masks 121 a and 121 b are removed.

Through the above steps, the thin film transistor 130A which isillustrated in FIGS. 1A to 1C and the thin film transistor 130B and acapacitor 131 which are illustrated in FIGS. 2A to 2C can be formed, andthe thin film transistor 130B and the capacitor 131 can havelight-transmitting properties. Further, the source wiring portion andthe gate wiring portion in the pixel portion can have light-transmittingproperties.

Note that the conditions of the etching may be set as appropriate sothat the lower semiconductor layers 113 a and 113 e are left. As each ofthe materials of the semiconductor layers 113 a and 113 e and thematerials of the conductive layers 119 a, 119 b, 119 e, 119 g, and 119h, a material with high etching selectivity is preferably used. Forexample, a metal oxide material containing Sn (e.g., SnZnO_(x) (x>0) orSnGaZnO_(x) (x>0)) may be used as each of the materials of thesemiconductor layers, and ITO or the like may be used as each of thematerials of the conductive layers 119 a, 119 b, 119 e, 119 g, and 119h. When the light-blocking conductive layer is removed, part of thelight-transmitting conductive layer (for example, a surface portionwhich is in contact with the light-blocking conductive layer) is alsoremoved in some cases. Therefore, for example, the thickness of theconductive layers 119 a and 119 b is often larger than the thickness ofthe conductive layers 119 e, 119 g, and 119 h.

Next, as illustrated in FIG. 9B and FIG. 11B, the insulating layer 123is formed over the thin film transistors 130A and 130B and the capacitor131. The insulating layer 123 can be formed to have a single-layerstructure or a layered structure. When the insulating layer 123 isformed to have a layered structure, the transmittance of each of filmsis preferably high enough. The insulating layer 123 functions as a filmwhich protects the thin film transistor from an impurity or the like.Further, the insulating layer 123 can function as a film for smoothingunevenness due to the thin film transistor, the capacitor, the wiring,or the like and for flattening a surface where the thin film transistor,the capacitor, the wiring, or the like is formed.

In particular, since the thin film transistor 130B and the capacitor 131in the pixel portion can be formed as light-transmitting elements, it isadvantageous to flatten an upper portion where these elements are formedby smoothing unevenness due to the thin film transistor 130B, thecapacitor 131, the wiring, or the like in order to use the region wherethese elements are formed as a display region.

The insulating layer 123 is preferably formed using a film containingsilicon nitride. A silicon nitride film is preferable because it ishighly effective in blocking impurities. Alternatively, the insulatinglayer 123 is preferably formed using a film containing an organicmaterial. As the organic material, acrylic, polyimide, polyamide, or thelike is preferable. Such an organic material is preferable because ofhigh functionality of flattening unevenness. Therefore, in the casewhere the insulating layer 123 is formed to have a layered structure ofa silicon nitride film and a film of an organic material, it ispreferable to provide the silicon nitride film on a lower side and thefilm containing an organic material on an upper side.

Further, before the formation of the insulating layer 123, an oxideinsulating film can be formed in contact with the semiconductor layer113 a and the semiconductor layer 113 e, for example. With the provisionof the oxide insulating film, the carrier concentration of thesemiconductor layers can be lowered.

In this case, the oxide insulating film has a thickness of at least 1 nmor larger and can be formed by a method by which an impurity such aswater or hydrogen are not mixed into the oxide insulating film, such assputtering, as appropriate. The substrate temperature at the time ofdeposition is in the range of room temperature to 300° C. A siliconoxide film can be deposited by sputtering in a rare gas (typicallyargon) atmosphere, an oxygen atmosphere, or an atmosphere including arare gas (typically argon) and oxygen. Further, a silicon oxide targetor a silicon target can be used as a target. For example, silicon oxidecan be deposited using a silicon target in an atmosphere includingoxygen and nitrogen by sputtering. The oxide insulating film which isformed in contact with the oxide semiconductor layer whose resistance islowered by dehydration or dehydrogenation is formed using an inorganicinsulating film which does not contain an impurity such as water, ahydrogen ion, or OH⁻ and blocks entry of such an impurity from theoutside, typically a silicon oxide film, a silicon nitride oxide film,an aluminum oxide film, or an aluminum oxynitride film.

Next, heat treatment (preferably at 200 to 400° C., for example, 250 to350° C.) may be performed in an inert gas atmosphere or an oxygen gasatmosphere. With the heat treatment, heat is applied while grooves inthe semiconductor layer 113 a and the semiconductor layer 113 e are incontact with the oxide insulating film.

Through the above steps, heat treatment for dehydration ordehydrogenation is performed on the oxide semiconductor film afterdeposition to reduce the resistance, and then, the oxide semiconductorfilm is changed into a high-resistant source region or a high-resistantdrain region and part of the high-resistant drain region is selectivelymade to be in an oxygen-excess state. Accordingly, the channel formationregion overlapping with the gate electrode layer becomes intrinsic, andthe high-resistant source region which overlaps with the sourceelectrode layer and the high-resistant drain region which overlaps withthe drain electrode layer are formed in a self-aligning manner. Further,the entire oxide semiconductor layer becomes intrinsic and serves as anoxide semiconductor layer including a channel formation region.

Next, as illustrated in FIG. 9C and FIG. 11C, a conductive film 206 anda conductive film 207 are stacked over the insulating layer 123 bysputtering. These steps can be performed successively and sequentialsputtering can be performed using a multi-chamber. By successivelyforming the conductive film 206 and the conductive film 207, throughputis improved and contamination by an impurity or dust can be suppressed.

It is preferable that the transmittance of the conductive film 206 besufficiently high. Further, the transmittance of the conductive film 206is preferably higher than the transmittance of the conductive film 207.

The conductive film 206 can be formed to have a single-layer structureor a layered structure including one or a plurality of materials whichcan be used for the conductive film 102 illustrated in FIGS. 3A and 3Band FIGS. 4A and 4B.

The conductive film 206 is preferably formed using a material which issubstantially the same as the materials of the conductive film 102 andthe conductive film 114. Substantially the same material is a materialwhose element of a main component is the same. For example, in terms ofimpurities, the kind and the concentration of elements contained aredifferent from each other in some cases. When the conductive film 206 isformed using the material which is substantially the same as thematerials of the conductive film 102 and the conductive film 114 bysputtering or evaporation in this manner, there is an advantage that thematerial can be shared among the conductive films 206, 102, and 114.When the material can be shared, the same manufacturing apparatus can beused, manufacturing steps can proceed smoothly, and throughput can beimproved, which leads to reduction in cost.

It is preferable that the resistance value of the conductive film 207 besufficiently low and the conductivity of the conductive film 207 besufficiently high. In addition, the resistance value of the conductivefilm 206 is preferably higher than the resistance value of theconductive film 207.

The conductive film 207 can be formed to have a single-layer structureor a layered structure including one or a plurality of materials whichcan be used for the conductive film 103 illustrated in FIGS. 3A and 3Band FIGS. 4A and 4B. Further, the conductive film 206 is preferablyformed using a material which is different from that used for theconductive film 207. Alternatively, the conductive film 207 ispreferably formed to have a layered structure which is different fromthat of the light-blocking conductive film. This is because, inmanufacturing steps, temperatures of the conductive film 206 and theconductive film 207 are different from each other in many cases. Ingeneral, the conductive film 207 tends to have a higher temperature. Theconductive film 207 is preferably formed to have a single-layerstructure or a layered structure of a layer formed using a materialhaving low wiring resistance. The conductive film 206 is preferablyformed using a light-transmitting material.

Note that in the case where the conductive film 207 is formed over theconductive film 206, both the films react with each other in some cases.For example, in the case where a top surface (a surface which is incontact with the conductive film 207) of the conductive film 206 isformed using ITO and a bottom surface (a surface which is in contactwith the conductive film 206) of the conductive film 207 is formed usingaluminum, chemical reaction occurs therebetween in some cases.Therefore, in order to avoid such chemical reaction, a high-meltingpoint material is preferably used for the bottom surface (the surfacewhich is in contact with the conductive film 206) of the conductive film207. For example, as the high-melting point material, molybdenum (Mo),titanium (Ti), tungsten (W), neodymium (Nd), or the like can be used. Itis preferable to form the conductive film 207 as a multi-layer film withthe use of a material having a low resistance value over the film formedusing the high-melting point material. As the material having a lowresistance value, aluminum (Al), copper (Cu), silver (Ag), or the likecan be used. Such a material has a light-blocking property andreflectivity.

Next, as illustrated in FIG. 9D and FIG. 11D, resist masks 300 a and 300e are formed over the conductive film 207. The resist masks 300 a and300 e are resist masks having regions with different thicknesses, whichare obtained using a multi-tone mask. The thickness of the resist masks300 a provided in the driver circuit portion is larger than thethickness of the resist mask 300 e provided in the pixel portion.

Next, as illustrated in FIG. 10A and FIG. 12A, the conductive film 206and the conductive film 207 are etched using the resist masks 300 a and300 e. With the etching, the conductive layers 400 a and 400 e andconductive layers 105 a and 105 e can be formed.

Then, as illustrated in FIG. 10B and FIG. 12B, ashing is performed onthe resist masks 300 a and 300 e. For example, ashing or the like inwhich oxygen plasma is used may be performed. When the resist mask 300 ais reduced (downsized) by the ashing, a resist mask 116 a is formed andpart of the conductive layer 105 a is exposed. Further, with this ashingtreatment, the resist mask 300 e in the pixel portion, which has smallthickness, is removed, and the conductive layer 105 e is exposed. Inthis manner, with the use of the resist mask formed using the multi-tonemask, a resist mask is not additionally used, so that steps can besimplified.

Next, as illustrated in FIG. 10C and FIG. 12C, the conductive layer 105a is etched using the resist mask 116 a. Thus, the conductive layer 401a which is obtained by removal of part of the conductive layer 105 a isformed, and part of the conductive layers 400 a is exposed. Further, theconductive layer 105 e is removed and the conductive layer 400 e isexposed. Note that end portions of the conductive layer 400 a protrudefrom end portions of the conductive layer 401 a. Furthermore, the areasof conductive layers 400 a and 401 a are greatly different from eachother. That is, the area of the conductive layer 400 a is larger thanthe area of the conductive layer 401 a. After the etching, the resistmask 116 a is removed.

Next, as illustrated in FIG. 10D and FIG. 12D, an insulating layer 208is formed over the conductive layers 400 a and 400 e and the conductivelayer 401 a. The insulating layer 208 can be formed to have asingle-layer structure or a layered structure. When the insulating layer208 is formed to have a layered structure, the transmittance of each offilms is preferably high enough. The insulating layer 208 can functionas a film for smoothing unevenness due to the conductive layers 400 aand 400 e and the conductive layer 401 a and for flattening surfaces.That is, the insulating layer 208 can function as a planarization film.The insulating layer 208 is preferably formed using a film containingsilicon nitride. A silicon nitride film is preferable because it ishighly effective in blocking impurities. Alternatively, the insulatinglayer 208 is preferably formed using a film containing an organicmaterial. As the organic material, acrylic, polyimide, polyamide, or thelike is preferable. Such an organic material is preferable because ofhigh functionality of flattening unevenness. Therefore, in the casewhere the insulating layer 208 is formed to have a layered structure ofa silicon nitride film and a film of an organic material, it ispreferable to provide the silicon nitride film on a lower side and thefilm containing an organic material on an upper side.

Note that each of the insulating layer 123 and the insulating layer 208can have a function as a color filter. When a color filter is providedover the substrate 101, it is not necessary to provide a color filter ona counter substrate. Therefore, a margin for adjusting the positions ofthe two substrates is not necessary, which can facilitate manufacture ofa panel.

Next, a resist mask is formed over the insulating layer 208. Whenetching is performed using the resist mask, some of the insulating layer123 and the insulating layer 208 are removed so that a contact hole 117is formed.

Then, as illustrated in FIG. 12E, a conductive film is formed over theinsulating layer 123 and in the contact hole 117, and a resist mask isformed over the conductive film. When etching is performed using theresist mask, part of the conductive film is removed so that conductivelayers 124 e, 124 g, and 124 h are formed. The conductive film can beformed to have a single-layer structure or a layered structure. When theconductive film is formed to have a layered structure, the transmittanceof each of films is preferably high enough.

The conductive layers 124 e, 124 g, and 124 h can function as pixelelectrodes. Alternatively, the conductive layers 124 e, 124 g, and 124 hcan function as electrodes of the capacitor. Therefore, it is preferablethat the conductive layers 124 e, 124 g, and 124 h be formed using alight-transmitting material or a material having high transmittance.

The conductive layers 124 e, 124 g, and 124 h can be connected to thesource wiring, the source electrode layer, the gate wiring, the gateelectrode layer, the pixel electrode, the capacitor wiring, theelectrode of the capacitor, or the like through the contact hole 117.Therefore, the conductive layers 124 e, 124 g, and 124 h can function aswirings for connecting conductors to each other.

The conductive layers 124 e, 124 g, and 124 h are preferably formedusing a material which is substantially the same as the material of theconductive film 102. Alternatively, the conductive layers 124 e, 124 g,and 124 h are preferably formed using a material which is substantiallythe same as the material of the conductive film 114. Alternatively, theconductive layers 124 e, 124 g, and 124 h are preferably formed using amaterial which is substantially the same as the material of theconductive film 206. When the conductive layers 124 e, 124 g, and 124 hare formed using the material which is substantially the same as thematerial of the conductive film 102, 114, or 206 by sputtering orevaporation in this manner, there is an advantage that the material canbe shared. When the material can be shared, the same manufacturingapparatus can be used, manufacturing steps can proceed smoothly, andthroughput can be improved, which leads to reduction in cost.

Through the steps illustrated in FIGS. 3A to 3D, FIGS. 4A to 4D, FIGS.5A to 5D, FIGS. 6A to 6D, FIGS. 7A to 7C, FIGS. 8A to 8C, FIGS. 9A to9D, FIGS. 10A to 10D, FIGS. 11A to 11D, and FIGS. 12A to 12E, over thesame substrate, the thin film transistor 130A in the driver circuitportion and the thin film transistor 130B in the pixel portion can beseparately formed with the use of six masks. Further, the capacitor 131can be formed over the same substrate. The thin film transistors 130Band the capacitors 131 are arranged in matrix to correspond toindividual pixels. Thus, one of the substrates for manufacturing anactive matrix display device can be obtained. In this specification,such a substrate is referred to as an active matrix substrate forconvenience.

By the method for manufacturing a semiconductor device, which isillustrated in FIGS. 3A to 3D, FIGS. 4A to 4D, FIGS. 5A to 5D, FIGS. 6Ato 6D, FIGS. 7A to 7C, FIGS. 8A to 8C, FIGS. 9A to 9D, FIGS. 10A to 10D,FIGS. 11A to 11D, and FIGS. 12A to 12E, a light-transmitting conductivefilm is formed; a conductive film whose resistance value is lower thanthat of the light-transmitting conductive film is stacked over thelight-transmitting conductive film; and the stacked films areselectively etched using a multi-tone mask so that a gate electrodelayer, a source electrode layer, or a drain electrode layer of a thinfilm transistor in a driver circuit portion which is formed using thestack of the light-transmitting conductive film and the conductive filmwhose resistance value is lower than that of the light-transmittingconductive film, and a gate electrode layer, a source electrode layer,or a drain electrode layer of a thin film transistor in a pixel portionwhich is formed using the light-transmitting conductive film, areformed. Thus, without the increase in the number of masks, gateelectrode layers, source electrode layers, or drain electrode layerswith different structures can be separately formed in the driver circuitportion and the pixel portion. Therefore, the number of manufacturingsteps can be reduced, so that manufacturing cost can be reduced.

By the method for manufacturing a semiconductor device, which isillustrated in FIGS. 3A to 3D, FIGS. 4A to 4D, FIGS. 5A to 5D, FIGS. 6Ato 6D, FIGS. 7A to 7C, FIGS. 8A to 8C, FIGS. 9A to 9D, FIGS. 10A to 10D,FIGS. 11A to 11D, and FIGS. 12A to 12E, a light-transmitting conductivefilm is formed; a conductive film whose resistance value is lower thanthat of the light-transmitting conductive film is stacked over thelight-transmitting conductive film; and the stacked films areselectively etched using a multi-tone mask so that a gate wiring, asource wiring, or a different lead wiring of the thin film transistor inthe driver circuit portion which is formed using the stack of thelight-transmitting conductive film and the conductive film whoseresistance value is lower than that of the light-transmitting conductivefilm, and a gate wiring, a source wiring, or a different lead wiring ofthe thin film transistor in the pixel portion which is formed using thelight-transmitting conductive film, can be formed. Thus, without theincrease in the number of masks, gate wirings, source wirings, or otherlead wirings with different structures can be separately formed in thedriver circuit portion and the pixel portion. Therefore, the number ofmanufacturing steps can be reduced, so that manufacturing cost can bereduced.

By the method for manufacturing a semiconductor device, which isillustrated in FIGS. 3A to 3D, FIGS. 4A to 4D, FIGS. 5A to 5D, FIGS. 6Ato 6D, FIGS. 7A to 7C, FIGS. 8A to 8C, FIGS. 9A to 9D, FIGS. 10A to 10D,FIGS. 11A to 11D, and FIGS. 12A to 12E, a storage capacitor formed usinga light-transmitting conductive layer and a dielectric layer can beformed in the same step as the thin film transistor in the pixelportion. Thus, without the increase in the number of masks, the thinfilm transistor and the storage capacitor can be separately formed inthe pixel portion. Therefore, the number of manufacturing steps can bereduced, so that manufacturing cost can be reduced.

By the method for manufacturing a semiconductor device, which isillustrated in FIGS. 3A to 3D, FIGS. 4A to 4D, FIGS. 5A to 5D, FIGS. 6Ato 6D, FIGS. 7A to 7C, FIGS. 8A to 8C, FIGS. 9A to 9D, FIGS. 10A to 10D,FIGS. 11A to 11D, and FIGS. 12A to 12E, a conductive film whoseresistance value is lower than that of a light-transmitting conductivefilm is stacked over the light-transmitting conductive film; and forexample, the stacked films are selectively etched using a multi-tonemask so that a conductive layer which overlaps with a channel formationregion of the thin film transistor in the driver circuit portion whichis formed using the stack of the light-transmitting conductive film andthe conductive film whose resistance value is lower than that of thelight-transmitting conductive film, and a conductive layer whichoverlaps with a channel formation region of the thin film transistor inthe pixel portion which is formed using the light-transmittingconductive film, can be formed. The conductive layers which overlap withthe channel formation regions of the thin film transistors can functionas back gate electrode layers of the thin film transistors. By themethod for manufacturing a semiconductor device, which is illustrated inFIGS. 3A to 3D, FIGS. 4A to 4D, FIGS. 5A to 5D, FIGS. 6A to 6D, FIGS. 7Ato 7C, FIGS. 8A to 8C, FIGS. 9A to 9D, FIGS. 10A to 10D, FIGS. 11A to11D, and FIGS. 12A to 12E, without the increase in the number of masks,conductive layers with different structures can be separately formed inthe driver circuit portion and the pixel portion. Therefore, the numberof manufacturing steps can be reduced, so that manufacturing cost can bereduced.

Next, an example of the structure of the semiconductor device includinga pixel portion which is different from that in FIGS. 2A to 2C isdescribed with reference to FIGS. 13A and 13B. FIG. 13A is a top view ofa semiconductor device of this embodiment, and FIG. 13B is across-sectional view taken along line J-K in FIG. 13A. FIGS. 13A and 13Bdiffer from FIGS. 2A to 2C in that the area of a lower electrode of astorage capacitor portion is made larger and an upper electrode of thestorage capacitor portion is a pixel electrode 124. The size of thestorage capacitor portion is preferably larger than pixel pitch by 70%or more or 80% or more. In the following description, since thestructure except for the storage capacitor portion and the storagecapacitor wiring in FIGS. 13A and 13B is the same as that in FIGS. 2A to2C, detailed description thereof is omitted.

With such a structure, transmittance can be increased because the upperelectrode of the storage capacitor portion does not need to be formedwhen the source wiring and the source electrode layer and the drainelectrode layer are formed. In addition, the large storage capacitorportion with high transmittance can be formed. By forming the largestorage capacitor portion, even if the thin film transistor is turnedoff, a potential of the pixel electrode is easily held. Further, afeedthrough potential can be lowered. Furthermore, even if the largestorage capacitor portion is formed, the aperture ratio can be increasedand power consumption can be reduced. Moreover, since the insulatingfilm has two layers, interlayer short-circuit due to a pinhole or thelike in the insulating film can be prevented, unevenness of thecapacitor wiring can be reduced, and alignment disorder of liquidcrystals can be suppressed.

Next, an example of the structure of the semiconductor device, which isdifferent from that in FIGS. 2A to 2C, is described with reference toFIGS. 14A and 14B.

FIG. 14A is a top view of a semiconductor device of this embodiment, andFIG. 14B is a cross-sectional view taken along line K-L in FIG. 14A.FIGS. 14A and 14B differ from FIGS. 2A to 2C in that a lower electrodeof a storage capacitor portion is made larger, a capacitor wiring, agate wiring, and a source wiring are formed using light-transmittingconductive layers, and an upper electrode of the storage capacitorportion is made larger. The size of the storage capacitor portion ispreferably larger than pixel pitch by 70% or more or 80% or more. In thefollowing description, since the structure except for the storagecapacitor portion in FIGS. 14A and 14B is the same as that in FIGS. 2Ato 2C, detailed description thereof is omitted.

With such a structure, distortion in the waveform of a signal andvoltage drop due to wiring resistance can be suppressed because thecapacitor wiring can be formed using a material with a low resistancevalue and high conductivity. In addition, even if disorder of thealignment of liquid crystals is caused by unevenness due to a contacthole in the pixel electrode, light leakage can be prevented by thelight-blocking conductive layer in the capacitor wiring. Further, byforming the large storage capacitor, even if the thin film transistor isturned off, a potential of the pixel electrode is easily held.Furthermore, a feedthrough potential can be lowered. Moreover, even ifthe large storage capacitor is formed, the aperture ratio can beincreased and power consumption can be reduced.

Next, an example of the structure of the semiconductor device, which isdifferent from that in FIGS. 2A to 2C, is described with reference toFIGS. 15A and 15B. FIG. 15A is a top view of a semiconductor device ofthis embodiment, and FIG. 15B is a cross-sectional view taken along lineM-N in FIG. 15A. FIGS. 15A and 15B differ from FIGS. 2A to 2C in that alight-transmitting conductive layer which functions as a lower electrodeof a storage capacitor portion is made larger and a light-transmittingconductive layer which functions as an upper electrode of the storagecapacitor portion is made larger. The size of the storage capacitorportion is preferably larger than pixel pitch by 70% or more or 80% ormore. In the following description, since the structure except for thestorage capacitor portion in FIGS. 15A and 15B is the same as that inFIGS. 2A to 2C, detailed description thereof is omitted.

With such a structure, the large storage capacitor with hightransmittance can be formed. By forming the large storage capacitor,even if the thin film transistor is turned off, a potential of the pixelelectrode is easily held. Further, a feedthrough potential can belowered. Furthermore, even if the large storage capacitor is formed, theaperture ratio can be increased and power consumption can be reduced.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 2

According to one embodiment of the present invention, thin filmtransistors are formed, and a semiconductor device having a displayfunction (also referred to as a display device) can be manufacturedusing the thin film transistor in a pixel portion and the thin filmtransistor in a driver circuit. Further, when part or whole of a drivercircuit including a thin film transistor is formed over the substrate asa pixel portion including a thin film transistor, a system-on-panel canbe obtained.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement) or a light-emitting element (also referred to as alight-emitting display element) can be used. The light-emitting elementincludes, in its category, an element whose luminance is controlled bycurrent or voltage, and specifically includes an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Further, a display medium whose contrast is changed by electric action,such as electronic ink, can be used.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel. Further, an element substrate, whichcorresponds to one embodiment before the display element is completed ina manufacturing process of the display device, is provided with a meansfor supplying current to the display element in each of a plurality ofpixels. Specifically, the element substrate may be in a state in whichonly a pixel electrode of the display element is formed, a state afterthe formation of a conductive film serving as a pixel electrode andbefore etching of the conductive film so that the pixel electrode isformed, or any other states.

Note that a display device in this specification refers to an imagedisplay device or a light source (including a lighting device). Further,the display device includes the following modules in its category: amodule including a connector such as a flexible printed circuit (FPC), atape automated bonding (TAB) tape, or a tape carrier package (TCP); amodule having a TAB tape or a TCP which is provided with a printedwiring board at the end thereof; and a module having an integratedcircuit (IC) which is directly mounted on a display element by a chip onglass (COG) method.

The appearance and a cross section of a liquid crystal display panel,which is one embodiment of a semiconductor device, is described withreference to FIGS. 17A and 17B. FIG. 17A is a plan view of a panel inwhich thin film transistors 4010 and 4011 and a liquid crystal element4013 are sealed between a first substrate 4001 and a second substrate4006 with a sealant 4005. FIG. 17B is a cross-sectional view taken alongline Q-R in FIG. 17A.

Note that in each of the thin film transistors 4010 and 4011 illustratedin FIGS. 17A and 17B, end portions of a gate electrode layer, a gateinsulating layer, a semiconductor layer, a source electrode layer, and adrain electrode layer are tapered. In this manner, by tapering the endportions of the layers, coverage with layers formed on and in contactwith the layers can be improved, disconnection can be prevented, and theyield of the semiconductor device can be improved. Note that thisembodiment is not limited to this structure. The end portion of the gateelectrode layer, the gate insulating layer, the semiconductor layer, thesource electrode layer, or the drain electrode layer is not necessarilytapered. Alternatively, one or more of the layers may be tapered.

The sealant 4005 is provided so as to surround a pixel portion 4002, asignal line driver circuit 4003, and a scan line driver circuit 4004which are provided over the first substrate 4001. The second substrate4006 is provided over the pixel portion 4002, the signal line drivercircuit, and the scan line driver circuit 4004. Thus, the pixel portion4002, the signal line driver circuit 4003, and the scan line drivercircuit 4004 are sealed together with a liquid crystal 4008 by the firstsubstrate 4001, the sealant 4005, and the second substrate 4006. Notethat in this embodiment, an example is described in which the pixelportion 4002, the signal line driver circuit 4003, and the scan linedriver circuit 4004 are formed over the first substrate 4001: however,the signal line driver circuit 4003 or the scan line driver circuit 4004may be formed over a substrate separately prepared with the use of athin film transistor including a single crystal semiconductor or apolycrystalline semiconductor so as to be attached onto the firstsubstrate 4001. FIGS. 17A and 17B illustrate examples of thin filmtransistors formed using oxide semiconductors in the pixel portion 4002,the signal line driver circuit 4003, and the scan line driver circuit4004.

The pixel portion 4002, the signal line driver circuit 4003, and thescan line driver circuit 4004 provided over the first substrate 4001include a plurality of thin film transistors. FIG. 17B illustrates thethin film transistor 4010 included in the pixel portion 4002 and thethin film transistor 4011 included in the signal line driver circuit4003. The thin film transistors 4010 and 4011 correspond to thin filmtransistors formed using n-type semiconductor layers. Although a storagecapacitor portion is not illustrated in the pixel portion 4002, thestorage capacitor portion illustrated in FIGS. 2A to 2C, FIGS. 13A and13B, FIGS. 14A and 14B, and FIGS. 15A and 15B can be formed.

As described above, in a driver circuit portion, the gate wiring whichis electrically connected to the gate electrode layer of the thin filmtransistor and includes the gate electrode layer is formed by stacking alight-transmitting conductive layer and a light-blocking conductivelayer having high conductivity in that order, and the source wiringwhich is electrically connected to the source electrode layer or thedrain electrode layer of the thin film transistor and includes thesource electrode layer is formed by stacking a light-transmittingconductive layer and a light-blocking conductive layer having highconductivity in that order. In the pixel portion, the gate wiring whichis electrically connected to the gate electrode layer of the thin filmtransistor and includes the gate electrode layer is formed using only alight-transmitting conductive layer, and the source wiring which iselectrically connected to the source electrode layer or the drainelectrode layer of the thin film transistor and includes the sourceelectrode layer is formed using only a light-transmitting conductivelayer. In other words, the gate wiring which is electrically connectedto the gate electrode layer of the thin film transistor and includes thegate electrode layer in the pixel portion is formed using part of thelight-transmitting conductive layer which is included in the gate wiringelectrically connected to the gate electrode layer of the thin filmtransistor and includes the gate electrode layer in the driver circuitportion; and the source wiring which is electrically connected to thesource electrode layer or the drain electrode layer of the thin filmtransistor and includes the source electrode layer in the pixel portionis formed using part of the light-transmitting conductive layer which isincluded in the source wiring electrically connected to the sourceelectrode layer or the drain electrode layer of the thin film transistorand includes the source electrode layer in the driver circuit portion.

In the pixel portion, the gate wiring including the gate electrodelayer, the source wiring including the source electrode layer, and aback gate are each formed by stacking a light-transmitting conductivelayer and a light-blocking conductive layer having high conductivity inthat order, so that wiring resistance is reduced and power consumptioncan be reduced. Further, since a light-blocking conductive film is usedas one of conductive films included in the back gate when the back gateis provided in the pixel portion, a space between pixels can be shieldedfrom light. That is, a space between pixels can be shielded from lightwithout the use of a black matrix.

By forming the storage capacitor portion in the pixel portion with theuse of the light-transmitting conductive layer as described above, theaperture ratio can be improved. In addition, by forming the storagecapacitor portion with the use of the light-transmitting conductivelayer, the storage capacitor portion can be made large, so that apotential of a pixel electrode can be easily held even if the thin filmtransistor is turned off.

In addition, reference numeral 4013 denotes a liquid crystal element. Apixel electrode 4030 included in the liquid crystal element 4013 iselectrically connected to the thin film transistor 4010 through a wiring4040. A counter electrode 4031 of the liquid crystal element 4013 isformed on the second substrate 4006. A portion where the pixel electrode4030, the counter electrode 4031, and the liquid crystal 4008 overlapwith each other corresponds to the liquid crystal element 4013.

Note that each of the first substrate 4001 and the second substrate 4006can be formed using glass, metal (typically, stainless steel), ceramics,or plastics. As plastics, a fiberglass-reinforced plastic (FRP) plate, apoly(vinyl fluoride) (PVF) film, a polyester film, or an acrylic resinfilm can be used. Alternatively, a sheet with a structure in whichaluminum foil is sandwiched between PVF films or polyester films can beused.

A spherical spacer 4035 is provided in order to control the distance (acell gap) between the pixel electrode 4030 and the counter electrode4031. Note that a spacer obtained by selective etching of an insulatingfilm may be used.

A variety of signals and potentials supplied to the signal line drivercircuit 4003 which is formed separately, the scan line driver circuit4004, or the pixel portion 4002 are supplied from an FPC 4018 throughlead wirings 4014 and 4015.

In this embodiment, a connection terminal electrode 4016 is formed usingthe same conductive film as the pixel electrode 4030 included in theliquid crystal element 4013. Further, the lead wiring 4015 is formedusing the same conductive film as the wiring 4040.

The connection terminal electrode 4016 is electrically connected to aterminal of the FPC 4018 through an anisotropic conductive film 4019.

Note that although not illustrated, the liquid crystal display deviceillustrated in this embodiment may include an alignment film.Alternatively, a liquid crystal exhibiting a blue phase for which analignment film is not used may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while the temperature of a cholestericliquid crystal is raised. Since the blue phase is generated within anarrow range of temperature, a liquid crystal composition containing achiral agent at 5 wt % or more is used for the liquid crystal 4008 inorder to improve the temperature range. A liquid crystal compositionincluding a liquid crystal exhibiting a blue phase and a chiral agenthas a short response time of 1 msec or less and is optically isotropic;thus, alignment treatment is not necessary and viewing angle dependenceis small.

Note that this embodiment can also be applied to a transflective liquidcrystal display device in addition to a transmissive liquid crystaldisplay device.

In an example of the liquid crystal display device, a polarizer isprovided on an outer surface of the substrate (on the viewer side), anda coloring layer (a color filter) and an electrode layer used for adisplay element are sequentially provided on all inner surface of thesubstrate; however, the polarizer may be provided on the inner surfaceof the substrate. The layered structure of the polarizer and thecoloring layer is not limited to that in this embodiment and may be setas appropriate depending on the materials of the polarizer and thecoloring layer or conditions of the manufacturing process. Further, alight-blocking film serving as a black matrix may be provided except ina display portion.

A conductive layer 4050 is provided over part of the insulating layer4021 so as to overlap with a channel formation region of an oxidesemiconductor layer in the thin film transistor 4011 for the drivercircuit. The conductive layer 4050 is provided so as to overlap with thechannel formation region of the oxide semiconductor layer, whereby theamount of change in the threshold voltage of the thin film transistor4011 before and after BT test can be reduced. Further, a potential ofthe conductive layer 4050 may be the same as or different from that of agate electrode layer of the thin film transistor 4011. The conductivelayer 4050 can function also as a second gate electrode layer.Alternatively, the potential of the conductive layer 4050 may be GND or0 V, or the conductive layer 4050 may be in a floating sate. Note that aconductive layer 4060 may be formed using a light-transmittingconductive material so as to overlap with a channel formation region ofan oxide semiconductor layer of the thin film transistor 4010 in thepixel portion.

The insulating layer 4021 is formed as a planarization insulating film.The insulating layer 4021 may be formed using a material and a methodwhich are similar to those of the planarization insulating layer 454described in Embodiment 1, and an organic material having heatresistance, such as polyimide, acrylic, benzocyclobutene, polyamide, orepoxy, can be used. Other than such an organic material, it is possibleto use a low-dielectric constant material (a low-k material), asiloxane-based resin, PSG (phosphosilicate glass), BPSG(borophosphosilicate glass), or the like. Note that the insulating layer4021 may be formed by stacking a plurality of insulating films formedusing these materials.

Note that a siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include an organic group (e.g.,an alkyl group or an aryl group) or a fluoro group as a substituent.Further, the organic group may include a fluoro group.

There is no particular limitation to the method of forming theinsulating layer 4021. The insulating layer 4021 can be formed,depending on the material, by a method such as sputtering, an SOGmethod, a spin coating method, a dipping method, a spray coating method,a droplet discharge method (e.g., an inkjet method, screen printing, oroffset printing), or a tool such as a doctor knife, a roll coater, acurtain coater, or a knife coater. A baking step of the insulating layer4021 also serves as annealing of the semiconductor layer, whereby asemiconductor device can be efficiently manufactured.

Each of the pixel electrode 4030 and the counter electrode 4031 can beformed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (ITO), indium zinc oxide, orindium tin oxide to which silicon oxide is added.

Alternatively, a conductive composition including a conductive highmolecule (also referred to as a conductive polymer) can be used for eachof the pixel electrode 4030 and the counter electrode 4031. The pixelelectrode formed using the conductive composition preferably has a sheetresistance of lower than or equal to 10000 ohms per square and atransmittance of higher than or equal to 70% at a wavelength of 550 nm.The sheet resistance is preferably lower. Further, the resistivity ofthe conductive high molecule included in the conductive composition ispreferably lower than or equal to 0.1 Ω·cm.

As the conductive high molecule, a so-called π-electron conjugatedconductive high molecule can be used. Examples are polyaniline and aderivative thereof, polypyrrole and a derivative thereof, polythiopheneand a derivative thereof, a copolymer of two or more kinds of thesematerials, and the like.

Further, a variety of signals and potentials are supplied from an FPC4018 to the signal line driver circuit 4003 which is separately formed,the scan line driver circuit 4004, or the pixel portion 4002.

A connection terminal electrode 4016 is formed using the same conductivefilm as the pixel electrode 4030 included in the liquid crystal element4013. The lead wiring 4015 is formed using the same conductive film assource electrode layers and drain electrode layers of the thin filmtransistors 4010 and 4011.

The connection terminal electrode 4016 is electrically connected to aterminal of the FPC 4018 through an anisotropic conductive film 4019.

FIGS. 17A and 17B illustrate an example in which the signal line drivercircuit 4003 is formed separately and mounted on the first substrate4001; however, this embodiment is not limited to this structure. Thescan line driver circuit may be separately formed and mounted, or onlypart of the signal line driver circuit or part of the scan line drivercircuit may be separately formed and mounted.

FIG. 18 illustrates an example of a liquid crystal display module whichis formed as a semiconductor device by using a TFT substrate 2600manufactured by the manufacturing method disclosed in thisspecification.

FIG. 18 illustrates an example of a liquid crystal display module, inwhich the TFT substrate 2600 and a counter substrate 2601 are fixed witha sealant 2602, and a pixel portion 2603 including a TFT and the like, adisplay element 2604 including a liquid crystal layer, and a coloringlayer 2605 are provided between the substrates to form a display region.The coloring layer 2605 is necessary to perform color display. In an RGBsystem, coloring layers corresponding to colors of red, green, and blueare provided for pixels. Polarizers 2606 and 2607 and a diffusion plate2613 are provided outside the TFT substrate 2600 and the countersubstrate 2601. A light source includes a cold cathode fluorescent lamp2610 and a reflector 2611. A circuit board 2612 is connected to a wiringcircuit portion 2608 of the TFT substrate 2600 by a flexible wiringboard 2609 and includes an external circuit such as a control circuit ora power source circuit. The polarizer and the liquid crystal layer maybe stacked with a retardation plate therebetween.

For the liquid crystal display module, a TN (twisted nematic) mode, anIPS (in-plane-switching) mode, an FFS (fringe field switching) mode, anMVA (multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASM (axially symmetric aligned micro-cell) mode, anOCB (optically compensated birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode,or the like can be used.

Through the above steps, a highly reliable liquid crystal display panelcan be manufactured as a semiconductor device.

This embodiment can be combined with any of the structures described inthe other embodiments as appropriate.

Embodiment 3

In this embodiment, an example of electronic paper is described as oneembodiment of a semiconductor device.

The semiconductor device may be used for electronic paper in whichelectronic ink is driven by an element which is electrically connectedto a switching element. Electronic paper is also referred to as anelectrophoretic display device (an electrophoretic display) and hasadvantages of the same level of readability as plain paper, lower powerconsumption than other display devices, and reduction in thickness andweight.

Electrophoretic displays can have various modes. Electrophoreticdisplays contain a plurality of microcapsules dispersed in a solvent ora solute, each of which contains first particles which are positivelycharged and second particles which are negatively charged. By applyingan electric field to the microcapsules, the particles in themicrocapsules move in opposite directions and only the color of theparticles gathering on one side is displayed. Note that the firstparticles and the second particles contain pigments and do not movewithout an electric field. Further, the first particles and the secondparticles have different colors (which may be colorless).

In this manner, an electrophoretic display utilizes a so-calleddielectrophoretic effect by which a substance having a high dielectricconstant moves to a high-electric field region. Note that theelectrophoretic display does not need a polarizer which is needed in aliquid crystal display device.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. This electronic ink can be printed ona surface of glass, plastics, cloth, paper, or the like. Further, colordisplay can be realized with a color filter or particles includingpigments.

When a plurality of the above microcapsules are arranged as appropriateover an active matrix substrate so as to be sandwiched between twoelectrodes, an active matrix display device is completed, and displaycan be performed by application of an electric field to themicrocapsules. For example, the active matrix substrate including thethin film transistor in Embodiment 1 can be used.

Note that each of the first particles and the second particles in themicrocapsules may be formed using one of a conductive material, aninsulating material, a semiconductor material, a magnetic material, aliquid crystal material, a ferroelectric material, an electroluminescentmaterial, an electrochromic material, and a magnetophoretic material, ora composite material of any of these materials.

FIG. 19 illustrates active matrix electronic paper as an example of asemiconductor device. A thin film transistor 581 used in thesemiconductor device can be formed in a manner which is similar to thatof the thin film transistor described in Embodiment 1 and is a highlyreliable thin film transistor including an oxide semiconductor layer.

The electronic paper in FIG. 19 is an example of a display device usinga twisting ball display system. The twisting ball display system refersto a method in which spherical particles each colored in black and whiteare provided between a first electrode layer and a second electrodelayer which are electrode layers used for a display element, and apotential difference is generated between the first electrode layer andthe second electrode layer in order to control the orientation of thespherical particles, so that display is performed.

The thin film transistor 581 formed over a substrate 580 is abottom-gate thin film transistor and is covered with an insulating film586 which is in contact with the oxide semiconductor layer and aninsulating film 585 which is in contact with the insulating film 586. Asource electrode layer or a drain electrode layer of the thin filmtransistor 581 which is sealed between the substrate 580 and a substrate596 is in contact with a first electrode layer 587 through an openingformed in the insulating film 585, whereby the thin film transistor 581is electrically connected to the first electrode layer 587. Sphericalparticles 589 are provided between the first electrode layer 587 and asecond electrode layer 588 formed on the substrate 596. Each of thespherical particles 589 includes a black region 590 a, a white region590 b, and a cavity 594 filled with liquid around the black region 590 aand the white region 590 b. A space around the spherical particles 589is filled with a filler 595 such as a resin (see FIG. 19). Note that theinsulating film 585 which covers the thin film transistor 581 may haveeither a single-layer structure or a layered structure. The firstelectrode layer 587 corresponds to a pixel electrode, and the secondelectrode layer 588 corresponds to a common electrode. The secondelectrode layer 588 is electrically connected to a common potential lineprovided over the same substrate as the thin film transistor 581. Withthe use of a common connection portion, the second electrode layer 588and the common potential line can be electrically connected to eachother through conductive particles provided between the substrates 580and 596.

Note that in the thin film transistor 581 illustrated in FIG. 19, endportions of a gate electrode layer, a gate insulating layer, asemiconductor layer, the source electrode layer, and the drain electrodelayer are tapered. In this manner, by tapering the end portions of thelayers, coverage with layers formed on and in contact with the layerscan be improved, disconnection can be prevented, and the yield of thesemiconductor device can be improved. Note that this embodiment is notlimited to this structure. The end portion of the gate electrode layer,the gate insulating layer, the semiconductor layer, the source electrodelayer, or the drain electrode layer is not necessarily tapered.Alternatively, one or more of the layers may be tapered.

It is possible to use an electrophoretic element instead of the elementusing the twisting ball. A microcapsule having a diameter ofapproximately 10 to 200 μm, in which transparent liquid, positivelycharged white microparticles, and negatively charged blackmicroparticles are encapsulated, is used. In the microcapsule providedbetween a first electrode layer and a second electrode layer, when anelectric field is applied by the first electrode layer and the secondelectrode layer, the white microparticles and the black microparticlesmove in opposite directions, so that white or black can be displayed. Adisplay element utilizing this principle is an electrophoretic displayelement, and a device including an electrophoretic display element iscalled an electronic paper in general. The electrophoretic displayelement has higher reflectance than a liquid crystal display element;thus, an auxiliary light is unnecessary, power consumption is low, and adisplay portion can be recognized even in a dim environment. Inaddition, even when power is not supplied to the display portion, animage which has been displayed once can be held. Thus, a displayed imagecan be held even if a semiconductor device having a display function(which may be referred to simply as a display device or a semiconductordevice including a display device) is disconnected from a power source.

Through the above steps, highly reliable electronic paper can bemanufactured as a semiconductor device.

This embodiment can be combined with any of the structures described inthe other embodiments as appropriate.

Embodiment 4

In this embodiment, an example of a light-emitting display device isdescribed as a semiconductor device. Here, a light-emitting elementutilizing electroluminescence is described as a display element includedin a display device. Light-emitting elements utilizingelectroluminescence are classified according to whether a light-emittingmaterial is an organic compound or an inorganic compound. In general,the former is referred to as an organic EL element, and the latter isreferred to as an inorganic EL element.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are injected from a pair of electrodes intoa layer containing a light-emitting organic compound, and current flows.These carriers (electrons and holes) are recombined, so that thelight-emitting organic compound emits light. Due to such a mechanism,the light-emitting element is referred to as a current-excitationlight-emitting element.

Inorganic EL elements are classified according to their elementstructures into dispersion-type inorganic EL elements and thin-filminorganic EL elements. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission which utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is interposed between dielectriclayers, which are further interposed between electrodes, and its lightemission mechanism is localized type light emission which utilizesinner-shell electron transition of metal ions. Note that here, anorganic EL element is used as a light-emitting element.

Next, structures of light-emitting elements are described with referenceto FIGS. 20A to 20C. Here, cross-sectional structures of pixels aredescribed using n-channel driving TFTs as an example. TFTs 7001, 7011,and 7021 used in semiconductor devices in FIGS. 20A to 20C can be formedin a manner which is similar to that of the thin film transistordescribed in any of the above embodiments.

In order to extract light from a light-emitting element, at least one ofan anode and a cathode is transparent. Here, the term “transparent”means that at least transmittance at the wavelength of emitted light issufficiently high. As a method for extracting light, there are a topemission method (a top extraction method) by which light is extractedfrom a side opposite to a substrate where a thin film transistor and alight-emitting element are formed, a bottom emission method (a bottomextraction method) by which light is extracted from the substrate side,a dual emission method (a dual extraction method) by which light isextracted from both the substrate side and the side opposite to thesubstrate, and the like.

A top-emission-type light-emitting element is described with referenceto FIG. 20A.

FIG. 20A is a cross-sectional view of a pixel when light is emitted froma light-emitting element 702 to an anode 705 side. Here, thelight-emitting element 702 is formed over a light-transmittingconductive layer 707 which is electrically connected to the driving TFT701, and a light-emitting layer 704 and the anode 705 are stacked inthat order over a cathode 703. For the cathode 703, a conductive filmwhich has a low work function and reflects light can be used. Forexample, the cathode 703 is preferably formed using Ca, Al, Mg—Ag,Al—Li, or the like. The light-emitting layer 704 may be formed usingeither a single layer or a plurality of layers stacked. In the casewhere the light-emitting layer 704 is formed using a plurality oflayers, an electron injection layer, an electron transport layer, alight-emitting layer, a hole transport layer, and a hole injection layerare preferably stacked in that order over the cathode 703; however,needless to say, it is not necessary to form all of these layers. Theanode 705 is formed using a light-transmitting conductive material. Forexample, a light-transmitting conductive material, such as indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, indium tin oxide (ITO), indium zinc oxide, or indium tinoxide to which silicon oxide is added, may be used.

A structure in which the light-emitting layer 704 is sandwiched betweenthe cathode 703 and the anode 705 can be called the light-emittingelement 702. In the case of the pixel illustrated in FIG. 20A, light isemitted from the light-emitting element 702 to the anode 705 side asindicated by an arrow. The structure of the light-emitting element 702may be a microcavity structure. Accordingly, it is possible to selectwavelength to be extracted, so that color purity can be improved. Notethat in that case, the thickness of layers included in thelight-emitting element 702 is set in accordance with the wavelength tobe extracted. Further, an electrode is preferably formed using amaterial with predetermined reflectivity.

An insulating layer containing silicon nitride, silicon oxide, or thelike may be formed over the anode 705. Accordingly, deterioration of thelight-emitting element can be suppressed.

Next, a bottom-emission-type light-emitting element is described withreference to FIG. 20B.

FIG. 20B is a cross-sectional view of a pixel when light is emitted froma light-emitting element 712 to a cathode 713 side. Here, the cathode713 of the light-emitting element 712 is formed over alight-transmitting conductive film 717 which is electrically connectedto the driving TFT 711, and a light-emitting layer 714 and an anode 715are stacked in that order over the cathode 713. Note that when the anode715 has a light-transmitting property, a light-blocking film 716 may beprovided so as to cover the anode 715. For the cathode 713, a conductivematerial having a low work function can be used as in the case of FIG.20A. Note that the thickness of the cathode 7013 is set such that lightis transmitted therethrough (preferably, approximately 5 to 30 nm). Forexample, an aluminum film with a thickness of approximately 20 nm can beused for the cathode 713. The light-emitting layer 714 may be formedusing either a single layer or a plurality of layers stacked, as in FIG.20A. The anode 715 dose not necessarily transmit light, but may beformed using a light-transmitting conductive material as in the case ofFIG. 20A. The light-blocking film 716 can be formed using a metal or thelike which reflects light; however, this embodiment is not limited tothis. Note that when the light-blocking film 716 has a function ofreflecting light, light extraction efficiency can be improved.

A structure in which the light-emitting layer 714 is sandwiched betweenthe cathode 713 and the anode 715 can be called the light-emittingelement 712. In the case of the pixel illustrated in FIG. 20B, light isemitted from the light-emitting element 712 to the cathode 713 side asindicated by an arrow. The structure of the light-emitting element 712may be a microcavity structure. Further, an insulating layer may beformed over the anode 715.

Next, a dual-emission-type light-emitting element is described usingFIG. 20C.

In FIG. 20C, a cathode 723 of a light-emitting element 722 is formedover a light-transmitting conductive film 727 which is electricallyconnected to the driving TFT 721, and a light-emitting layer 724 and ananode 725 are stacked in that order over the cathode 723. For thecathode 723, a conductive material having a low work function can beused as in the case of FIG. 20A. The thickness of the cathode 7023 isset such that light is transmitted therethrough. For example, a20-nm-thick aluminum film can be used for the cathode 723. As in thecase of FIG. 20A, the light-emitting layer 724 may be formed usingeither a single layer or a plurality of layers stacked. As in FIG. 20A,the anode 725 can be formed using a light-transmitting conductivematerial.

A structure in which the light-emitting layer 724 is sandwiched betweenthe cathode 723 and the anode 725 can be called the light-emittingelement 722. In the case of the pixel illustrated in FIG. 20C, light isemitted from the light-emitting element 722 to both the anode 725 sideand the cathode 723 side as indicated by arrows. The structure of thelight-emitting element 722 may be a microcavity structure. Further, aninsulating layer may be formed over the anode 725.

Note that although organic EL elements are described here as thelight-emitting elements, inorganic EL elements can be provided as thelight-emitting elements. The example is described here in which a thinfilm transistor (a TFT for driving a light-emitting element) whichcontrols the driving of a light-emitting element is electricallyconnected to the light-emitting element; however, a structure may beemployed in which a TFT for current control is connected between thedriving TFT and the light-emitting element.

Note that the structure of the semiconductor device in this embodimentis not limited to the structures illustrated in FIGS. 20A to 20C, andcan be modified in various ways.

Next, appearance and a cross section of a light-emitting display panel(also referred to as a light-emitting panel), which corresponds to oneembodiment of a semiconductor device, are described with reference toFIGS. 21A and 21B. FIG. 21A is a top view of a panel in which thin filmtransistors 4509 and 4510 and a light-emitting element 4511 which areformed over a first substrate 4501 are sealed between the firstsubstrate 4501 and a second substrate 4506 with a sealant 4505. FIG. 21Bis a cross-sectional view taken along line S-T in FIG. 21A.

The sealant 4505 is provided so as to surround a pixel portion 4502,signal line driver circuits 4503 a and 4503 b, and scan line drivercircuits 4504 a and 4504 b which are provided over the first substrate4501. In addition, a second substrate 4506 is provided over the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b. In other words, the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b are sealed together with afiller 4507, by the first substrate 4501, the sealant 4505, and thesecond substrate 4506. It is preferable that the panel be packaged(sealed) with a protective film (e.g., an attachment film or anultraviolet curable resin film) or a cover material, which has highair-tightness and causes less degasification, so that the panel is notexposed to the external air, in this manner.

Further, the pixel portion 4502, the signal line driver circuits 4503 aand 4503 b, and the scan line driver circuits 4504 a and 4504 b whichare provided over the first substrate 4501 each include a plurality ofthin film transistors, and the thin film transistor 4510 included in thepixel portion 4502 and the thin film transistor 4509 included in thesignal line driver circuit 4503 a are illustrated in FIG. 21B.

As each of the thin film transistors 4509 and 4510, any of the thin filmtransistors in Embodiments 1 to 3 can be used. Note that in thisembodiment, the thin film transistors 4509 and 4510 are n-channel thinfilm transistors.

Further, reference numeral 4511 denotes a light-emitting element. Afirst electrode layer 4517 which is a pixel electrode of thelight-emitting element 4511 is electrically connected to a sourceelectrode layer or a drain electrode layer of the thin film transistor4510. Note that although the light-emitting element 4511 has a layeredstructure of the first electrode layer 4517, a second electrode layer4512, an electroluminescent layer 4513, and a third electrode layer4514, the structure of the light-emitting element 4511 is not limited tothe structure described in this embodiment. The above structure can bechanged as appropriate depending on a direction in which light isextracted from the light-emitting element 4511, or the like.

A partition 4520 is formed using an organic resin film, an inorganicinsulating film, or organic polysiloxane. It is particularly preferablethat the partition 4520 be formed using a photosensitive material tohave an opening over the first electrode layer 4517 so that a sidewallof the opening is formed as an inclined surface with continuouscurvature.

The electroluminescent layer 4513 may be formed using either a singlelayer or a plurality of layers stacked.

A protective film may be formed over the third electrode layer 4514 andthe partition 4520 in order to prevent oxygen, hydrogen, water, carbondioxide, or the like from entering the light-emitting element 4511. Asthe protective film, a silicon nitride film, a silicon nitride oxidefilm, a DLC film, or the like can be formed.

In addition, a variety of signals are supplied from FPCs 4518 a and 4518b to the signal line driver circuits 4503 a and 4503 b, the scan linedriver circuits 4504 a and 4504 b, the pixel portion 4502, or the like.

In this embodiment, a connection terminal electrode 4515 is formed usingthe same conductive film as the first electrode layer 4517 of thelight-emitting element 4511, and a terminal electrode 4516 is formedusing the same conductive film as the source electrode layers and thedrain electrode layers of the thin film transistors 4509 and 4510.

The connection terminal electrode 4515 is electrically connected to aterminal of the FPC 4518 a through an anisotropic conductive film 4519.

The substrate located in the direction in which light is extracted fromthe light-emitting element 4511 needs to have a light-transmittingproperty with respect to visible light. For a substrate having alight-transmitting property with respect to visible light, a glassplate, a plastic plate, a polyester film, an acrylic film, or the likecan be used.

Further, as well as an inert gas such as nitrogen or argon, anultraviolet curable resin or a thermosetting resin can be used as thefiller 4507. For example, PVC (polyvinyl chloride), acrylic, polyimide,an epoxy resin, a silicone resin, PVB (polyvinyl butyral), EVA (ethylenevinyl acetate), or the like can be used. In this embodiment, an examplewhere nitrogen is used for the filler is described.

If needed, an optical film such as a polarizer, a circular polarizer(including an elliptical polarizer), a retardation plate (a quarter-waveplate or a half-wave plate), or a color filter, may be provided asappropriate on a light-emitting surface of the light-emitting element.Further, anti-reflection treatment may be performed on a surface. Forexample, anti-glare treatment can be performed by which reflected lightcan be diffused by projections and depressions on the surface so thatglare can be reduced.

The signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b may be formed using a single crystalsemiconductor or a polycrystalline semiconductor over a substrateseparately prepared. Alternatively, only the signal line driver circuitsor some of the signal line driver circuits, or only the scan line drivercircuits or some of the scan line driver circuits may be separatelyformed and mounted. This embodiment is not limited to the structureillustrated in FIGS. 21A and 21B.

Through the above steps, a high-performance light-emitting displaydevice (display panel) can be manufactured.

Next, a structure and operation of a pixel to which digital time ratiograyscale driving can be applied are described. FIGS. 22A and 22B eachillustrate an example of a pixel structure to which digital time ratiograyscale driving can be applied. Here, one pixel includes two n-channelthin film transistors each having an oxide semiconductor layer (anIn—Ga—Zn—O-based non-single-crystal film) for a channel formationregion.

In FIG. 22A, pixel 6400 includes a switching thin film transistor 6401,a thin film transistor 6402 for driving a light-emitting element(hereinafter referred to as the driving thin film transistor 6402), alight-emitting element 6404, and a capacitor 6403. A gate of theswitching thin film transistor 6401 is connected to a scan line 6406. Afirst electrode (one of a source electrode layer and a drain electrodelayer) of the switching thin film transistor 6401 is connected to asignal line 6405. A second electrode (the other of the source electrodelayer and the drain electrode layer) of the switching thin filmtransistor 6401 is connected to a gate of the driving thin filmtransistor 6402. The gate of the driving thin film transistor 6402 isconnected to a power supply line 6407 through the capacitor 6403. Afirst electrode of the driving thin film transistor 6402 is connected tothe power supply line 6407. A second electrode of the driving thin filmtransistor 6402 is connected to a first electrode (a pixel electrode) ofthe light-emitting element 6404. A second electrode of thelight-emitting element 6404 corresponds to a common electrode 6408.

Note that as for the relation of potentials of the second electrode (onthe common electrode 6408 side) and the first electrode (on the powersupply line 6407 side) of the light-emitting element 6404, one of thepotentials may be set higher than the other. In the light-emittingdisplay device, a potential difference between a high potential and alow potential is applied to the light-emitting element 6404 and currentflows to the light-emitting element 6404, so that the light-emittingelement 6404 emits light. Therefore, each potential is set so that thepotential difference between the high potential and the low potential ishigher than or equal to the threshold voltage of the light-emittingelement 6404.

Note that gate capacitance of the driving thin film transistor 6402 maybe used as a substitute for the capacitor 6403, so that the capacitor6403 can be eliminated. The gate capacitance of the driving thin filmtransistor 6402 may be formed with a channel region and the gateelectrode layer.

Here, in the case of a voltage-input voltage driving method, a videosignal is input to the gate of the driving thin film transistor 6402 sothat the driving thin film transistor 6402 is sufficiently turned on orturned off. That is, the driving thin film transistor 6402 operates in alinear region.

In addition, by making input signals vary, analog grayscale driving canbe performed using the pixel structure illustrated in FIG. 22A. Forexample, when an analog video signal is used, current corresponding tothe video signal can be supplied to the light-emitting element 6404 andanalog grayscale driving can be performed. The video signal ispreferably a signal with which the driving thin film transistor 6402operates in a saturation region.

Further, a potential of the power supply line 6407 may be changed in apulsed manner. In this case, it is preferable to employ a structureillustrated in FIG. 22B.

Further, in the structure in FIG. 22A, the potential of the secondelectrode of the light-emitting element 6404 in a given pixel is oftenthe same as a potential of a second electrode in another pixel (apotential of the common electrode 6408); alternatively, cathodes may bepatterned for each pixel and connected to driving transistors.

Note that one embodiment of the disclosed invention is not construed asbeing limited to the pixel structures illustrated in FIGS. 22A and 22B.For example, a switch, a resistor, a capacitor, a thin film transistor,a logic circuit, or the like may be added to the pixel illustrated inFIGS. 22A and 22B.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 5

A semiconductor device can be applied to electronic paper. Electronicpaper can be used for electronic devices in all fields for displayingdata. For example, electronic paper can be used for electronic bookreaders (e-book readers), posters, advertisement in vehicles such astrains, display portions in a variety of cards such as credit cards, andthe like. Examples of such electronic devices are illustrated in FIGS.23A and 23B and FIG. 24.

FIG. 23A illustrates a poster 2631 formed using electronic paper. In thecase where an advertising medium is printed paper, the advertisement isreplaced by hands; however, by using the electronic paper, theadvertising display can be changed in a short time. Further, stableimages can be obtained without display defects. Note that the poster maytransmit and receive data wirelessly.

FIG. 23B illustrates an advertisement 2632 in a vehicle such as a train.In the case where an advertising medium is paper, the advertisement isreplaced by hands; however, by using the electronic paper, theadvertising display can be changed in a short time with less manpower.Further, stable images can be obtained without display defects. Notethat the poster may transmit and receive data wirelessly.

FIG. 24 illustrates an e-book reader 2700. For example, the e-bookreader 2700 includes two housings 2701 and 2703. The housings 2701 and2703 are combined with each other with a hinge 2711 so that the e-bookreader 2700 can be opened and closed with the hinge 2711 as an axis.With such a structure, the e-book reader 2700 can be operated like apaper book.

A display portion 2705 is incorporated in the housing 2701, and adisplay portion 2707 is incorporated in the housing 2703. The displayportions 2705 and 2707 may display one image or different images. In thecase where the display portion 2705 and 2707 display different images,for example, a display portion on the right side (the display portion2705 in FIG. 24) can display text and a display portion on the left side(the display portion 2707 in FIG. 24) can display images.

FIG. 24 illustrates an example in which the housing 2701 includes anoperation portion and the like. For example, the housing 2701 includes apower switch 2721, operation keys 2723, a speaker 2725, and the like.With the operation keys 2723, pages can be turned. Note that a keyboard,a pointing device, or the like may be provided on a surface of thehousing, on which the display portion is provided. Further, an externalconnection terminal (e.g., an earphone terminal, a USB terminal, or aterminal which can be connected to a variety of cables such as USBcables), a recording medium insertion portion, or the like may beprovided on a back surface or a side surface of the housing.Furthermore, the e-book reader 2700 may function as an electronicdictionary.

Further, the e-book reader 2700 may transmit and receive datawirelessly. Through wireless communication, desired book data or thelike can be purchased and downloaded from an electronic book server.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 6

In this embodiment, structures and operation of a pixel which can beused in a liquid crystal display device are described. Note that as theoperation mode of a liquid crystal element in this embodiment, a TN(twisted nematic) mode, an IPS (in-plane-switching) mode, an FFS (fringefield switching) mode, an MVA (multi-domain vertical alignment) mode, aPVA (patterned vertical alignment) mode, an ASM (axially symmetricaligned microcell) mode, an OCB (optically compensated birefringence)mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(anti-ferroelectric liquid crystal) mode, or the like can be used.

FIG. 25A illustrates an example of a pixel structure which can be usedin the liquid crystal display device. A pixel 5080 includes a transistor5081, a liquid crystal element 5082, and a capacitor 5083. A gate of thetransistor 5081 is electrically connected to a wiring 5085. A firstterminal of the transistor 5081 is electrically connected to a wiring5084. A second terminal of the transistor 5081 is electrically connectedto a first terminal of the liquid crystal element 5082. A secondterminal of the liquid crystal element 5082 is electrically connected toa wiring 5087. A first terminal of the capacitor 5083 is electricallyconnected to the first terminal of the liquid crystal element 5082. Asecond terminal of the capacitor 5083 is electrically connected to awiring 5086. Note that a first terminal of a thin film transistor is oneof a source and a drain, and a second terminal of the thin filmtransistor is the other of the source and the drain. That is, when thefirst terminal of the thin film transistor is the source, the secondterminal of the thin film transistor is the drain. In a similar manner,when the first terminal of the thin film transistor is the drain, thesecond terminal of the thin film transistor is the source.

The wiring 5084 can serve as a signal line. The signal line is a wiringfor transmitting signal voltage, which is input from the outside of thepixel, to the pixel 5080. The wiring 5085 can serve as a scan line. Thescan line is a wiring for controlling on/off of the transistor 5081. Thewiring 5086 can serve as a capacitor line. The capacitor line is awiring for applying predetermined voltage to the second terminal of thecapacitor 5083. The transistor 5081 can serve as a switch. The capacitor5083 can serve as a storage capacitor. The storage capacitor is acapacitor with which the signal voltage is continuously applied to theliquid crystal element 5082 even when the switch is off. The wiring 5087can serve as a counter electrode. The counter electrode is a wiring forapplying predetermined voltage to the second terminal of the liquidcrystal element 5082. Note that the function of each wiring is notlimited to this, and each wiring can have a variety of functions. Forexample, by changing voltage applied to the capacitor line, voltageapplied to the liquid crystal element can be adjusted. Note that it isacceptable as long as the transistor 5081 serves as a switch, and thetransistor 5081 may be either a p-channel transistor or an n-channeltransistor.

FIG. 25B illustrates an example of a pixel structure which can be usedin the liquid crystal display device. The example of the pixel structureillustrated in FIG. 25B is the same as that in FIG. 25A except that thewiring 5087 is eliminated and the second terminal of the liquid crystalelement 5082 and the second terminal of the capacitor 5083 areelectrically connected to each other. The example of the pixel structureillustrated in FIG. 25B can be particularly used in the case of using ahorizontal electric field mode (including an IPS mode and an FFS mode)liquid crystal element. This is because in the horizontal electric fieldmode liquid crystal element, the second terminal of the liquid crystalelement 5082 and the second terminal of the capacitor 5083 can be formedover the same substrate, so that it is easy to electrically connect thesecond terminal of the liquid crystal element 5082 and the secondterminal of the capacitor 5083 to each other. With the pixel structureillustrated in FIG. 25B, the wiring 5087 can be eliminated, so that amanufacturing process can be simplified and manufacturing cost can bereduced.

A plurality of pixel structures illustrated in FIG. 25A or FIG. 25B canbe arranged in a matrix. Thus, a display portion of a liquid crystaldisplay device is formed, and a variety of images can be displayed. FIG.25C illustrates a circuit structure in the case where a plurality ofpixel structures illustrated in FIG. 25A are arranged in a matrix. FIG.25C is a circuit diagram illustrating four pixels among a plurality ofpixels included in the display portion. A pixel arranged in an i-thcolumn and a j-th row (each of i and j is a natural number) isrepresented as a pixel 5080 _(—) i, j, and a wiring 5084 _(—) j, awiring 5085 _(—) j, and a wiring 5086 _(—) j are electrically connectedto the pixel 5080 _(—) i, j. In a similar manner, a wiring 5084 _(—)i+1, the wiring 5085 _(—) j, and the wiring 5086 _(—) j are electricallyconnected to a pixel 5080 _(—) i+1, j. In a similar manner, the wiring5084 _(—) i, a wiring 5085 _(—) j+1, and a wiring 5086 _(—) j+1 areelectrically connected to a pixel 5080 _(—) i, j+1. In a similar manner,the wiring 5084 _(—) i+1, the wiring 5085 _(—) j+1, and the wiring 5086_(—) j+1 are electrically connected to a pixel 5080 _(—) j+1, j+1. Notethat each wiring can be used in common with a plurality of pixels in thesame row or the same column. In the pixel structure illustrated in FIG.25C, the wiring 5087 is a counter electrode, which is used by all thepixels in common; therefore, the wiring 5087 is not indicated by thenatural number i or j. Note that since the pixel structure in FIG. 25Bcan also be used, the wiring 5087 is not required even in a structurewhere the wiring 5087 is provided and can be eliminated when anotherwiring serves as the wiring 5087, for example.

The pixel structure in FIG. 25C can be driven by a variety of methods.In particular, when the pixels are driven by a method called AC drive,deterioration (burn-in) of the liquid crystal element can be suppressed.FIG. 25D is a timing chart of voltage applied to each wiring in thepixel structure in FIG. 25C in the case where dot inversion driving,which is a kind of AC drive, is performed. By the dot inversion driving,flickers seen when the AC drive is performed can be suppressed.

In the pixel structure in FIG. 25C, a switch in a pixel electricallyconnected to the wiring 5085 _(—) j is selected (in an on state) in aj-th gate selection period in one frame period and is not selected (inan off state) in the other periods. Then, a (j+1)th gate selectionperiod is provided after the j-th gate selection period. By performingsequential scanning in this manner, all the pixels are sequentiallyselected in one frame period. In the timing chart of FIG. 25D, theswitch in the pixel is selected when the level of voltage is high (highlevel), and the switch is not selected when the level of the voltage islow (low level). Note that this is the case where the thin filmtransistor in each pixel is an n-channel transistor. In the case ofusing a p-channel thin film transistor, a relationship between voltageand a selection state is opposite to that in the case of using ann-channel thin film transistor.

In the timing chart illustrated in FIG. 25D, in the j-th gate selectionperiod in a k-th frame (k is a natural number), positive signal voltageis applied to the wiring 5084 _(—) i used as a signal line, and negativesignal voltage is applied to the wiring 5084 _(—) i+1. Then, in the(j+1)th gate selection period in the k-th frame, negative signal voltageis applied to the wiring 5084 _(—) i, and positive signal voltage isapplied to the wiring 5084 _(—) i+1. After that, signals whosepolarities are inverted every gate selection period are alternatelysupplied to the signal line. Accordingly, in the k-th frame, thepositive signal voltage is applied to the pixels 5080 _(—) i, j and 5080i+1, j+1, and the negative signal voltage is applied to the pixels 5080_(—) i+1, j and 5080 _(—) i, j+1. Then, in a (k+1)th frame, signalvoltage whose polarity is opposite to that of the signal voltage writtenin the k-th frame is written to each pixel. Accordingly, in the (k+1)thframe, the positive signal voltage is applied to the pixels 5080 _(—)i+1, j and 5080 _(—) i, j+1, and the negative signal voltage is appliedto the pixels 5080 _(—) i, j and 5080 _(—) i+1, j+1. In this manner, thedot inversion driving is a driving method by which signal voltage whosepolarity is different between adjacent pixels is applied in the sameframe and the polarity of the signal voltage for the pixel is invertedevery one frame. By the dot inversion driving, flickers seen when theentire or part of an image to be displayed is uniform can be suppressedwhile deterioration of the liquid crystal element is suppressed. Notethat voltage applied to all the wirings 5086 including the wirings 5086_(—) j and 5086 _(—) j+1 can be fixed voltage. Note that although onlythe polarity of the signal voltage for the wirings 5084 is illustratedin the timing chart, the signal voltage can actually have a variety oflevels in the polarity illustrated. Note that here, the case where thepolarity is inverted per dot (per pixel) is described; however, thisembodiment is not limited to this, and the polarity can be inverted pera plurality of pixels. For example, the polarity of signal voltage to bewritten is inverted per two gate selection periods, so that powerconsumed in writing signal voltage can be reduced. Alternatively, thepolarity can be inverted per column (source line inversion) or per row(gate line inversion).

Note that fixed voltage may be applied to the second terminal of thecapacitor 5083 in the pixel 5080 in one frame period. Here, since thelevel of voltage applied to the wiring 5085 used as a scan line is lowlevel in most of one frame period, which means that substantiallyconstant voltage is applied to the wiring 5085; therefore, the secondterminal of the capacitor 5083 in the pixel 5080 may be connected to thewiring 5085. FIG. 25E illustrates an example of a pixel structure whichcan be used in the liquid crystal display device. Compared to the pixelstructure in FIG. 25C, a feature of the pixel structure in FIG. 25E liesin that the wiring 5086 is eliminated and the second terminal of thecapacitor 5083 in the pixel 5080 and the wiring 5085 in the previous roware electrically connected to each other. Specifically, in the rangeillustrated in FIG. 25E, the second terminals of the capacitors 5083 inthe pixels 5080 _(—) i, j+1 and 5080_i+1, j+1 are electrically connectedto the wiring 5085 _(—) j. By electrically connecting the secondterminal of the capacitor 5083 in the pixel 5080 and the wiring 5085 inthe previous row to each other in this manner, the wiring 5086 can beeliminated, so that the aperture ratio of the pixel can be increased.Note that the second terminal of the capacitor 5083 may be connected tothe wiring 5085 in another row instead of in the previous row. Note thatthe pixel structure in FIG. 25E can be driven by a driving method whichis similar to that in the pixel structure in FIG. 25C.

Note that voltage applied to the wiring 5084 used as a signal line canbe lowered by using the capacitor 5083 and the wiring electricallyconnected to the second terminal of the capacitor 5083. A pixelstructure and a driving method in this case are described with referenceto FIGS. 25F and 25G. Compared to the pixel structure in FIG. 25A, afeature of the pixel structure in FIG. 25F lies in that two wirings 5086are provided per pixel column, and in adjacent pixels, one wiring iselectrically connected to every other second terminal of the capacitors5083 and the other wiring is electrically connected to the remainingevery other second terminal of the capacitors 5083 in the pixel 5080.Note that two wirings 5086 are referred to as a wiring 5086-1 and awiring 5086-2. Specifically, in the range illustrated in FIG. 25F, thesecond terminal of the capacitor 5083 in the pixel 5080 _(—) i, j iselectrically connected to a wiring 5086-1 _(—) j; the second terminal ofthe capacitor 5083 in the pixel 5080 _(—) i+1, j is electricallyconnected to a wiring 5086-2 _(—) j; the second terminal of thecapacitor 5083 in the pixel 5080 _(—) i, j+1 is electrically connectedto a wiring 5086-2 _(—) j+1; and the second terminal of the capacitor5083 in the pixel 5080 _(—) i+1, j+1 is electrically connected to awiring 5086-1 _(—) j+1.

For example, when positive signal voltage is written to the pixel 5080_(—) i, j in the k-th frame as illustrated in FIG. 25G, the wiring5086-1 _(—) j becomes a low level, in the j-th gate selection period andis changed to a high level after the j-th gate selection period. Then,the wiring 5086-1 _(—) j is kept at a high level in one frame period,and after negative signal voltage is written in the j-th gate selectionperiod in the (k+1)th frame, the wiring 5086-1 _(—) j is changed to ahigh level. In this manner, voltage of the wiring which is electricallyconnected to the second terminal of the capacitor 5083 is changed in apositive direction after positive signal voltage is written to thepixel, so that voltage applied to the liquid crystal element can bechanged in the positive direction by a predetermined level. That is,signal voltage written to the pixel can be lowered by the predeterminedlevel, so that power consumed in signal writing can be reduced. Notethat when negative signal voltage is written in the j-th gate selectionperiod, voltage of the wiring which is electrically connected to thesecond terminal of the capacitor 5083 is changed in a negative directionafter negative signal voltage is written to the pixel. Thus, voltageapplied to the liquid crystal element can be changed in the negativedirection by a predetermined level, and the signal voltage written tothe pixel can be reduced as in the case of the positive polarity. Inother words, as for the wiring which is electrically connected to thesecond terminal of the capacitor 5083, different wirings are preferablyused for a pixel to which positive signal voltage is applied and a pixelto which negative signal voltage is applied in the same row of the sameframe. FIG. 25F illustrates an example in which the wiring 5086-1 iselectrically connected to the pixel to which positive signal voltage isapplied in the k-th frame and the wiring 5086-2 is electricallyconnected to the pixel to which negative signal voltage is applied inthe k-th frame. Note that this is just an example, and for example, inthe case of using a driving method by which pixels to which positivesignal voltage is written and pixels to which negative signal voltage iswritten appear every two pixels, it is preferable to perform electricalconnections with the wirings 5086-1 and 5086-2 alternately every twopixels. Further, in the case where signal voltage of the same polarityis written to all the pixels in one row (gate line inversion), onewiring 5086 may be provided per row. In other words, in the pixelstructure in FIG. 25C, the driving method by which signal voltagewritten to a pixel is lowered as described with reference to FIGS. 25Fand 25G can be used.

Next, a pixel structure and a driving method which are preferably usedparticularly in the case where the mode of a liquid crystal element is avertical alignment (VA) mode typified by an MVA mode and a PVA mode. TheVA mode has advantages such as no rubbing step in manufacture, littlelight leakage at the time of black display, and low driving voltage, buthas a problem in that image quality is decreased (the viewing angle isnarrower) when a screen is seen from all oblique angle. In order towiden the viewing angle in the VA mode, a pixel structure where onepixel includes a plurality of subpixels as illustrated in FIGS. 26A and26B is effective. Pixel structures illustrated in FIGS. 26A and 26B areexamples of the case where the pixel 5080 includes two subpixels (asubpixel 5080-1 and a subpixel 5080-2). Note that the number ofsubpixels in one pixel is not limited to two and can be other numbers.The viewing angle can be further widened as the number of subpixelsbecomes larger. A plurality of subpixels can have the same circuitstructure. Here, all the subpixels have the circuit structureillustrated in FIG. 25A. Note that the first subpixel 5080-1 includes athin film transistor 5081-1, a liquid crystal element 5082-1, and acapacitor 5083-1. The connection relation of each element is the same asthat in the circuit structure in FIG. 25A. In a similar manner, thesecond subpixel 5080-2 includes a transistor 5081-2, a liquid crystalelement 5082-2, and a capacitor 5083-2. The connection relation of eachelement is the same as that in the circuit structure in FIG. 25A.

The pixel structure in FIG. 26A includes, for two subpixels included inone pixel, two wirings 5085 (a wiring 5085-1 and a wiring 5085-2) usedas scan lines, one wiring 5084 used as a signal line, and one wiring5086 used as a capacitor line. When the signal line and the capacitorline are shared between two subpixels in this manner, the aperture ratiocan be improved. Further, since a signal line driver circuit can besimplified, manufacturing cost can be reduced. Furthermore, since thenumber of connections between a liquid crystal panel and a drivercircuit IC can be reduced, yield can be improved. The pixel structure inFIG. 26B includes, for two subpixels included in one pixel, one wiring5085 used as a scan line, two wirings 5084 (a wiring 5084-1 and a wiring5084-2) used as signal lines, and one wiring 5086 used as a capacitorline. When the scan line and the capacitor line are shared between twosubpixels in this manner, the aperture ratio can be improved. Further,since the total number of scan lines can be reduced, the length of eachgate line selection period can be sufficiently increased even in ahigh-definition liquid crystal panel, and appropriate signal voltage canbe written to each pixel.

FIGS. 26C and 26D illustrate an example in which the liquid crystalelement in the pixel structure in FIG. 26B is replaced with the shape ofa pixel electrode and the electrical connection of each element isschematically illustrated. In FIGS. 26C and 26D, an electrode 5088-1denotes a first pixel electrode, and an electrode 5088-2 denotes asecond pixel electrode. In FIG. 26C, the first pixel electrode 5088-1corresponds to a first terminal of the liquid crystal element 5082-1 inFIG. 26B, and the second pixel electrode 5088-2 corresponds to a firstterminal of the liquid crystal element 5082-2 in FIG. 26B. That is, thefirst pixel electrode 5088-1 is electrically connected to one of asource and a drain of the thin film transistor 5081-1, and the secondpixel electrode 5088-2 is electrically connected to one of a source anda drain of the thin film transistor 5081-2. Meanwhile, in FIG. 26D, theconnection relation between the pixel electrode and the thin filmtransistor is opposite to that in FIG. 26C. That is, the first pixelelectrode 5088-1 is electrically connected to one of the source and thedrain of the thin film transistor 5081-2, and the second pixel electrode5088-2 is electrically connected to one of the source and the drain ofthe thin film transistor 5081-1.

By alternately arranging a plurality of pixel structures illustrated inFIG. 26C and FIG. 26D in a matrix, special advantageous effects can beobtained. FIGS. 26E and 26F illustrate examples of the pixel structureand a driving method thereof. In the pixel structure in FIG. 26E, aportion corresponding to the pixels 5080 _(—) i, j and 5080 _(—) i+1,j+1 has the structure illustrated in FIG. 26C, and a portioncorresponding to the pixels 5080 _(—) i+1, j and 5080 _(—) i, j+1 hasthe structure illustrated in FIG. 26D. In this structure, by performingdriving like the timing chart illustrated in FIG. 26F, in the j-th gateselection period in the k-th frame, positive signal voltage is writtento the first pixel electrode in the pixel 5080 _(—) i, j and the secondpixel electrode in the pixel 5080 _(—) i+1, j, and negative signalvoltage is written to the second pixel electrode in the pixel 5080 _(—)i, j and the first pixel electrode in the pixel 5080 _(—) i+1, j. In the(j+1)th gate selection period in the k-th frame, positive signal voltageis written to the second pixel electrode in the pixel 5080 _(—) i, j+1and the first pixel electrode in the pixel 5080 _(—) i+1, j+1, andnegative signal voltage is written to the first pixel electrode in thepixel 5080 _(—) i, j+1 and the second pixel electrode in the pixel 5080_(—) i+1, j+1. In the (k+1)th frame, the polarity of signal voltage isinverted in each pixel. Thus, the polarity of voltage applied to thesignal line can be the same in one frame period while drivingcorresponding to dot inversion driving is realized in the pixelstructure including subpixels. Therefore, power consumed in writingsignal voltage to the pixels can be drastically reduced. Note thatvoltage applied to all the wirings 5086 including the wirings 5086 _(—)j and 5086 _(—) j+1 can be fixed voltage.

Further, by a pixel structure and a driving method illustrated in FIGS.26G and 26H, the level of signal voltage written to a pixel can belowered. In the structure, capacitors lines which are electricallyconnected to a plurality of subpixels included in each pixel aredifferent between the subpixels. That is, by using the pixel structureand the driving method illustrated in FIGS. 26G and 26H, subpixels towhich voltages having the same polarities are written in the same frameshare a capacitor line in the same row, and subpixels to which voltageshaving different polarities are written in the same frame use differentcapacitor lines in the same row. Then, when writing in each row isterminated, voltage of the capacitor lines is changed in the positivedirection in the subpixels to which positive signal voltage is written,and changed in the negative direction in the subpixels to which negativesignal voltage is written. Thus, the level of the signal voltage writtento the pixel can be lowered. Specifically, two wirings 5086 (the wirings5086-1 and 5086-2) used as capacitor lines are provided in each row. Thefirst pixel electrode in the pixel 5080 _(—) i, j and the wiring 5086-1;are electrically connected to each other through the capacitor. Thesecond pixel electrode in the pixel 5080 _(—) i, j and the wiring 5086-2_(—) j are electrically connected to each other through the capacitor.The first pixel electrode in the pixel 5080 _(—) i+1, j and the wiring5086-2 _(—) j are electrically connected to each other through thecapacitor. The second pixel electrode in the pixel 5080 _(—) i+1, j andthe wiring 5086-1 _(—) j are electrically connected to each otherthrough the capacitor. The first pixel electrode in the pixel 5080 _(—)i, j+1 and the wiring 5086-2 _(—) j+1 are electrically connected to eachother through the capacitor. The second pixel electrode in the pixel5080 _(—) i, j+1 and the wiring 5086-1 _(—) j+1 are electricallyconnected to each other through the capacitor. The first pixel electrodein the pixel 5080 _(—) i+1, j+1 and the wiring 5086-1 _(—) j+1 areelectrically connected to each other through the capacitor. The secondpixel electrode in the pixel 5080 _(—) i+1, j+1 and the wiring 5086-2_(—) j+1 are electrically connected to each other through the capacitor.Note that this is just an example, and for example, in the case of usinga driving method by which pixels to which positive signal voltage iswritten and pixels to which negative signal voltage is written appearevery two pixels, it is preferable to perform electrical connectionswith the wirings 5086-1 and 5086-2 alternately every two pixels.Further, in the case where signal voltage of the same polarity iswritten in all the pixels in one row (gate line inversion), one wiring5086 may be provided per row. In other words, in the pixel structure inFIG. 26E, the driving method by which signal voltage written to a pixelis lowered as described with reference to FIGS. 26G and 26H can be used.

Embodiment 7

Next, another structure example and a driving method of a display deviceare described. In this embodiment, the case of using a display deviceincluding a display element whose luminance response with respect tosignal writing is slow (response time is long) is described. In thisembodiment, a liquid crystal element is described as an example of thedisplay element with long response time. In this embodiment, a liquidcrystal element is illustrated as an example of the display element withlong response time. However, a display element in this embodiment is notlimited to this, and a variety of display elements whose luminanceresponse with respect to signal writing is slow can be used.

In a general liquid crystal display device, luminance response withrespect to signal writing is slow, and it sometimes takes more than oneframe period to complete the response even when signal voltage iscontinuously applied to a liquid crystal element. Moving images cannotbe displayed precisely by such a display element. Further, in the caseof active matrix driving, time for signal writing to one liquid crystalelement is only a period (one scan line selection period) obtained bydividing a signal writing cycle (one frame period or one subframeperiod) by the number of scan lines, and the liquid crystal elementcannot respond in such a short time in many cases. Therefore, most ofthe response of the liquid crystal element is performed in a periodduring which signal writing is not performed. Here, the dielectricconstant of the liquid crystal element is changed in accordance with thetransmittance of the liquid crystal element, and the response of theliquid crystal element in a period during which signal writing is notperformed means that the dielectric constant of the liquid crystalelement is changed in a state where electric charge is not exchangedwith the outside of the liquid crystal element (in a constant chargestate). In other words, in a formula wherecharge=(capacitance)·(voltage), the capacitance is changed in a statewhere the charge is constant. Accordingly, voltage applied to the liquidcrystal element is changed from voltage in signal writing, in accordancewith the response of the liquid crystal element. Therefore, in the casewhere the liquid crystal element whose luminance response with respectto signal writing is slow is driven by active matrix driving, voltageapplied to the liquid crystal element cannot theoretically reach thevoltage in signal writing.

In the display device in this embodiment, a signal level in signalwriting is corrected in advance (a correction signal is used) so that adisplay element can reach desired luminance within a signal writingcycle. Thus, the above problem can be solved. Further, since theresponse time of the liquid crystal element becomes shorter as thesignal level becomes higher, the response time of the liquid crystalelement can also be shorter by writing a correction signal. A drivingmethod by which such a correction signal is added is referred to asoverdrive. By overdrive in this embodiment, even when a signal writingcycle is shorter than a cycle for an image signal input to the displaydevice (an input image signal cycle T_(in)), the signal level iscorrected in accordance with the signal writing cycle, so that thedisplay element can reach desired luminance within the signal writingcycle. The case where the signal writing cycle is shorter than the inputimage signal cycle T_(in) is, for example, the case where one originalimage is divided into a plurality of subimages and the plurality ofsubimages are sequentially displayed in one frame period.

Next, an example of correcting a signal level in signal writing in adisplay device driven by active matrix driving is described withreference to FIGS. 27A and 27B. FIG. 27A is a graph schematicallyillustrating a time change in signal level in signal writing in onedisplay element, with the time as the horizontal axis and the signallevel in signal writing as the vertical axis. FIG. 27B is a graphschematically illustrating a time change in display level in one displayelement, with the time as the horizontal axis and the display level asthe vertical axis. Note that when the display element is a liquidcrystal element, the signal level in signal writing can be voltage, andthe display level can be the transmittance of the liquid crystalelement. In the following description, the vertical axis in FIG. 27A isregarded as the voltage, and the vertical axis in FIG. 27B is regardedas the transmittance. Note that in the overdrive in this embodiment, thesignal level may be other than the voltage (may be a duty ratio orcurrent, for example). Note that in the overdrive in this embodiment,the display level may be other than the transmittance (may be luminanceor current, for example). Liquid crystal elements are classified intotwo modes: a normally black mode in which black is displayed whenvoltage is 0 (e.g., a VA mode and an IPS mode), and a normally whitemode in which white is displayed when voltage is 0 (e.g., a TN mode andan OCB mode). The graph illustrated in FIG. 27B corresponds to both ofthe modes. The transmittance increases in the upper part of the graph inthe normally black mode, and the transmittance increases in the lowerpart of the graph in the normally white mode. That is, a liquid crystalmode in this embodiment may be either a normally black mode or anormally white mode. Note that timing of signal writing is representedon the time axis by dotted lines, and a period after signal writing isperformed until the next signal writing is performed is referred to as aretention period F_(i). In this embodiment, i is an integer and an indexfor representing each retention period. In FIGS. 27A and 27B, i is 0 to2; however, i can be an integer other than 0 to 2 (only the case where iis 0 to 2 is illustrated). Note that in the retention period F_(i),transmittance for realizing luminance corresponding to an image signalis denoted by T_(i), and voltage for providing the transmittance T_(i)in a constant state is denoted by V_(i). In FIG. 27A, a dashed line 5101represents a time change in voltage applied to the liquid crystalelement in the case where overdrive is not performed, and a solid line5102 represents a time change in voltage applied to the liquid crystalelement in the case where the overdrive in this embodiment is performed.In a similar manner, in FIG. 27B, a dashed line 5103 represents a timechange in transmittance of the liquid crystal element in the case whereoverdrive is not performed, and a solid line 5104 represents a timechange in transmittance of the liquid crystal element in the case wherethe overdrive in this embodiment is performed. Note that a differencebetween the desired transmittance T_(i) and the actual transmittance atthe end of the retention period F_(i) is referred to as an error α_(i).

It is assumed that, in the graph illustrated in FIG. 27A, both thedashed line 5101 and the solid line 5102 represent the case wheredesired voltage V₀ is applied in a retention period F₀; and in the graphillustrated in FIG. 27B, both the dashed line 5103 and the solid line5104 represent the case where desired transmittance T₀ is obtained. Inthe case where overdriving is not performed, desired voltage V₁ isapplied at the beginning of a retention period F₁ as shown by the dashedline 5101. As has been described above, a period for signal writing ismuch shorter than a retention period, and the liquid crystal element isin a constant charge state in most of the retention period. Accordingly,voltage applied to the liquid crystal element in the retention period F₁is changed along with a change in transmittance and is greatly differentfrom the desired voltage V₁ at the end of the retention period F₁. Inthis case, the dashed line 5103 in the graph of FIG. 27B is greatlydifferent from desired transmittance T₁. Accordingly, accurate displayof an image signal cannot be performed, so that image quality isdecreased. On the other hand, in the case where the overdrive in thisembodiment is performed, voltage V₁′ which is higher than the desiredvoltage V₁ is applied to the liquid crystal element at the beginning ofthe retention period F₁ as shown by the solid line 5102. That is, thevoltage V₁′ which is corrected from the desired voltage V₁ is applied tothe liquid crystal element at the beginning of the retention period F₁so that the voltage applied to the liquid crystal element at the end ofthe retention period F₁ is close to the desired voltage V₁ inanticipation of a gradual change in voltage applied to the liquidcrystal element in the retention period F₁. Thus, the desired voltage V₁can be accurately applied to the liquid crystal element. In this case,as shown by the solid line 5104 in the graph of FIG. 27B, the desiredtransmittance T₁ can be obtained at the end of the retention period F₁.In other words, the response of the liquid crystal element within thesignal writing cycle can be realized, despite the fact that the liquidcrystal element is in a constant charge state in most of the retentionperiod. Then, in a retention period F₂, the case where desired voltageV₂ is lower than V₁ is described. Also in that case, as in the retentionperiod F₁, voltage V₂′ which is corrected from the desired voltage V₂may be applied to the liquid crystal element at the beginning of theretention period F₂ so that the voltage applied to the liquid crystalelement at the end of the retention period F₂ is close to the desiredvoltage V₂ in anticipation of a gradual change in voltage applied to theliquid crystal element in the retention period F₂. Thus, as shown by thesolid line 5104 in the graph of FIG. 27B, desired transmittance T₂ canbe obtained at the end of the retention period F₂. Note that in the casewhere V_(i) is higher than V_(i−1) as in the retention period F₁, thecorrected voltage V_(i)′ is preferably corrected so as to be higher thandesired voltage V_(i). Further, when V_(i) is lower than V_(i−1) as inthe retention period F₂, the corrected voltage V_(i)′ is preferablycorrected so as to be lower than the desired voltage V_(i). Note that aspecific correction value can be derived by measuring responsecharacteristics of the liquid crystal element in advance. As a method ofrealizing overdrive in a device, a method by which a correction formulais formulated and included in a logic circuit, a method by which acorrection value is stored in a memory as a look-up table and is read asnecessary, or the like can be used.

Note that there are several limitations on realization of the overdrivein this embodiment in a device. For example, voltage correction has tobe performed in the range of the rated voltage of a source driver. Thatis, in the case where desired voltage is originally high and idealcorrection voltage exceeds the rated voltage of the source driver, notall the correction can be performed. Problems in such a case aredescribed with reference to FIGS. 27C and 27D. As in FIG. 27A, FIG. 27Cis a graph in which a time change in voltage in one liquid crystalelement is schematically illustrated as a solid line 5105 with the timeas the horizontal axis and the voltage as the vertical axis. As in FIG.27B, FIG. 27D is a graph in which a time change in transmittance of oneliquid crystal element is schematically illustrated as a solid line 5106with the time as the horizontal axis and the transmittance as thevertical axis. Note that since other references are similar to those inFIGS. 27A and 27B, description thereof is omitted. FIGS. 27C and 27Dillustrate a state where sufficient correction cannot be performedbecause the correction voltage V₁′ for realizing the desiredtransmittance T₁ in the retention period F₁ exceeds the rated voltage ofthe source driver; thus V₁′=V₁ has to be given. In this case, thetransmittance at the end of the retention period F₁ is deviated from thedesired transmittance T₁ by the error α₁. Note that the error α₁ isincreased only when the desired voltage is originally high; therefore, adecrease in image quality due to occurrence of the error α₁ is in theallowable range in many cases. However, as the error α₁ is increased, anerror in algorithm for voltage correction is also increased. In otherwords, in the algorithm for voltage correction, when it is assumed thatthe desired transmittance is obtained at the end of the retentionperiod, even though the error α₁ is increased, voltage correction isperformed on the basis that the error α₁ is small. Accordingly, theerror is included in correction in the following retention period F₂;thus, an error α₂ is also increased. Further, in the case where theerror α₂ is increased, the following error α₃ is further increased, forexample, and the error is increased, which results in a significantdecrease in image quality. In the overdrive in this embodiment, in orderto prevent the increase of errors in such a manner, when the correctionvoltage V_(i)′ exceeds the rated voltage of the source driver in theretention period F_(i), an error α_(i) at the end of the retentionperiod F_(i) is estimated, and the correction voltage in a retentionperiod F_(i+1) can be adjusted in consideration of the amount of theerror α_(i). Thus, even when the error α_(i) is increased, the effect ofthe error α_(i) on the error α_(i+1) can be minimized, so that theincrease of errors can be prevented. An example where the error α₂ isminimized in the overdrive in this embodiment is described withreference to FIGS. 27E and 27F. In a graph of FIG. 27E, a solid line5107 represents a time change in voltage in the case where thecorrection voltage V₂′ in the graph of FIG. 27C is further adjusted tobe correction voltage V₂″. A graph of FIG. 27F illustrates a time changein transmittance in the case where voltage is corrected in accordancewith the graph of FIG. 27E. The solid line 5106 in the graph of FIG. 27Dindicates that excessive correction (correction in the case where anerror is large) is caused by the correction voltage V₂′. On the otherhand, the solid line 5108 in the graph of FIG. 27F indicates thatexcessive correction is suppressed by the correction voltage V₂″ whichis adjusted in consideration of the error α₁ and the error α₂ isminimized. Note that a specific correction value can be derived frommeasuring response characteristics of the liquid crystal element inadvance. As a method of realizing overdrive in a device, a method bywhich a correction formula is formulated and included in a logiccircuit, a method by which a correction value is stored in a memory as alook-up table and read as necessary, or the like can be used. Further,such a method can be added separately from a portion for calculatingcorrection voltage V_(i)′ or can be included in the portion forcalculating correction voltage V_(i)′. Note that the amount ofcorrection of correction voltage V_(i)′ which is adjusted inconsideration of an error α_(i−1) (a difference with the desired voltageV_(i)) is preferably smaller than that of V_(i)′. That is,|V_(i)′″−V_(i)|<|V_(i)′−V_(i)| is preferable.

Note that the error α_(i) which is caused because ideal correctionvoltage exceeds the rated voltage of the source driver is increased as asignal writing cycle becomes shorter. This is because the response timeof the liquid crystal element needs to be shorter as the signal writingcycle becomes shorter, so that higher correction voltage is necessary.Further, as a result of an increase in correction voltage needed, thecorrection voltage exceeds the rated voltage of the source driver morefrequently, so that the large error α_(i) occurs more frequently.Therefore, it can be said that the overdrive in this embodiment becomesmore effective as the signal writing cycle becomes shorter.Specifically, the overdrive in this embodiment is significantlyeffective in the case of performing the following driving methods: adriving method by which one original image is divided into a pluralityof subimages and the plurality of subimages are sequentially displayedin one frame period, a driving method by which motion of an image isdetected from a plurality of images and an intermediate image of theplurality of images is generated and inserted between the plurality ofimages (so-called motion compensation frame rate doubling), and adriving method in which such driving methods are combined, for example.

Note that the rated voltage of the source driver has the lower limit inaddition to the upper limit described above. An example of the lowerlimit is the case where voltage which is lower than the voltage 0 cannotbe applied. In this case, since ideal correction voltage cannot beapplied as in the case of the upper limit described above, the errorα_(i) is increased. However, also in that case, the error α_(i) at theend of the retention period F_(i) is estimated, and the correctionvoltage in the retention period F_(i+1) can be adjusted in considerationof the amount of the error α_(i) in a manner similar to the abovemethod. Note that in the case where voltage which is lower than thevoltage 0 (negative voltage) can be applied as the rated voltage of thesource driver, the negative voltage may be applied to the liquid crystalelement as correction voltage. Thus, the voltage applied to the liquidcrystal element at the end of retention period F_(i) can be adjusted soas to be close to the desired voltage V_(i) in anticipation of a changein potential due to a constant charge state.

Note that in order to suppress deterioration of the liquid crystalelement, so-called inversion driving by which the polarity of voltageapplied to the liquid crystal element is periodically inverted can beperformed in combination with the overdrive. That is, the overdrive inthis embodiment includes the case where the overdrive is performed atthe same time as the inversion driving. For example, in the case wherethe length of the signal writing cycle is half of that of the inputimage signal cycle T_(in), when the length of a cycle for invertingpolarity is the same or substantially the same as that of the inputimage signal cycle T_(in), two sets of writing of a positive signal andtwo sets of writing of a negative signal are alternately performed. Thelength of the cycle for inverting polarity is made larger than that ofthe signal writing cycle in this manner, so that the frequency of chargeand discharge of a pixel can be reduced. Thus, power consumption can bereduced. Note that when the cycle for inverting polarity is made toolong, a defect in which luminance difference due to the difference ofpolarity is recognized as a flicker occurs in some cases; therefore, itis preferable that the length of the cycle for inverting polarity besubstantially the same as or smaller than that of the input image signalcycle T_(in).

Embodiment 8

Next, another structure example and a driving method of a display deviceare described. In this embodiment, a method is described by which animage for interpolating motion of an image input from the outside of adisplay device (an input image) is generated inside the display devicebased on a plurality of input images and the generated image (thegeneration image) and the input image are sequentially displayed. Notethat when an image for interpolating motion of an input image is ageneration image, motion of moving images can be made smooth, and adecrease in quality of moving images because of afterimages or the likedue to hold driving can be suppressed. Here, moving image interpolationis described below. Ideally, display of moving images is realized bycontrolling the luminance of each pixel in real time; however,individual control of pixels in real time has problems such as theenormous number of control circuits, space for wirings, and the enormousamount of input image data. Thus, it is difficult to realize theindividual control of pixels. Therefore, for display of moving images bya display device, a plurality of still images are sequentially displayedin a certain cycle so that display appears to be moving images. Thecycle (in this embodiment, referred to as an input image signal cycleand denoted by T_(in)) is standardized, and for example, 1/60 second inNTSC and 1/50 second in PAL. Such a cycle does not cause a problem ofmoving image display in a CRT, which is an impulsive display device.However, in a hold-type display device, when moving images conforming tothese standards are displayed without change, a defect in which displayis blurred because of afterimages or the like due to hold driving (holdblur) occurs. Since hold blur is recognized by discrepancy betweenunconscious motion interpolation due to human eyes tracking andhold-type display, the hold blur can be reduced by making the inputimage signal cycle shorter than that in conventional standards (bymaking the control closer to individual control of pixels in real time).However, it is difficult to reduce the length of the input image signalcycle because the standard needs to be changed and the amount of data isincreased. However, an image for interpolating motion of an input imageis generated inside the display device in response to a standardizedinput image signal, and display is performed while the generation imageinterpolates the input image, so that hold blur can be reduced without achange in the standard or an increase in the amount of data. Operationsuch that an image signal is generated inside the display device inresponse to an input image signal to interpolate motion of the inputimage is referred to as moving image interpolation.

By a method for interpolating moving images in this embodiment, motionblur can be reduced. The method for interpolating moving images in thisembodiment can include an image generation method and an image displaymethod. Further, by using a different image generation method and/or adifferent image display method for motion with a specific pattern,motion blur can be effectively reduced. FIGS. 28A and 28B are schematicdiagrams each illustrating an example of a method for interpolatingmoving images in this embodiment. FIGS. 28A and 28B each illustratetiming of treating each image by using the position of the horizontaldirection, with the time as the horizontal axis. A portion representedas “input” indicates timing at which an input image signal is input.Here, images 5121 and 5122 are focused as two images that are temporallyadjacent. An input image is input at an interval of the cycle T_(in).Note that the length of one cycle T_(in) is referred to as one frame orone frame period in some cases. A portion represented as “generation”indicates timing at which a new image is generated from an input imagesignal. Here, an image 5123 which is a generation image generated basedon the images 5121 and 5122 is focused. A portion represented as“display” indicates timing at which an image is displayed in the displaydevice. Note that images other than the focused images are onlyrepresented by dashed lines, and by treating such images in a mannersimilar to that of the focused images, the example of the method forinterpolating moving images in this embodiment can be realized.

In the example of the method for interpolating moving images in thisembodiment, as illustrated in FIG. 28A, a generation image which isgenerated based on two input images that are temporally adjacent isdisplayed in a period after one image is displayed until the other imageis displayed, so that moving image interpolation can be performed. Inthis case, a display cycle of a display image is preferably half of aninput cycle of the input image. Note that the display cycle is notlimited to this and can be a variety of display cycles. For example, inthe case where the length of the display cycle is shorter than half ofthat of the input cycle, moving images can be displayed more smoothly.Alternatively, in the case where the length of the display cycle islonger than half of that of the input cycle, power consumption can bereduced. Note that here, an image is generated based on two input imageswhich are temporally adjacent; however, the number of input imagesserving as a basis is not limited to two and can be other numbers. Forexample, when an image is generated based on three (may be more thanthree) input images which are temporally adjacent, a generation imagewith higher accuracy can be obtained as compared to the case where animage is generated based on two input images. Note that the displaytiming of the image 5121 is the same as the input timing of the image5122, that is, the display timing is one frame later than the inputtiming. However, display timing in the method for interpolating movingimages in this embodiment is not limited to this and can be a variety ofdisplay timings. For example, the display timing can be delayed withrespect to the input timing by more than one frame. Thus, the displaytiming of the image 5123 which is the generation image can be delayed,which allows enough time to generate the image 5123 and leads toreduction in power consumption and manufacturing cost. Note that whenthe display timing is delayed with respect to the input timing for along time, a period for holding an input image becomes longer, and thememory capacity which is necessary for holding the input image isincreased. Therefore, the display timing is preferably delayed withrespect to the input timing by approximately one to two frames.

Here, an example of a specific generation method of the image 5123 whichis generated based on the images 5121 and 5122 is described. It isnecessary to detect motion of an input image in order to interpolatemoving images. In this embodiment, a method called a block matchingmethod can be used in order to detect motion of an input image. Notethat this embodiment is not limited to this, and a variety of methods(e.g., a method for obtaining a difference of image data or a method ofusing Fourier transformation) can be used. In the block matching method,first, image data for one input image (here, image data of the image5121) is stored in a data storage means (e.g., a memory circuit such asa semiconductor memory or a RAM). Then, an image in the next frame(here, the image 5122) is divided into a plurality of regions. Note thatthe divided regions can have the same rectangular shapes as illustratedin FIG. 28A; however, the divided regions are not limited to them andcan have a variety of shapes (e.g., the shape or size varies dependingon images). After that, in each divided region, data is compared to theimage data in the previous frame (here, the image data of the image5121), which is stored in the data storage means, so that a region wherethe image data is similar to each other is searched. The example of FIG.28A illustrates that the image 5121 is searched for a region where datais similar to that of a region 5124 in the image 5122, and a region 5126is found. Note that a search range is preferably limited when the image5121 is searched. In the example of FIG. 28A, a region 5125 which isapproximately four times larger than the region 5124 is set as thesearch range. By making the search range larger than this, detectionaccuracy can be increased even in a moving image with high-speed motion.Note that search in an excessively wide range needs an enormous amountof time, which makes it difficult to realize detection of motion. Thus,the region 5125 has preferably approximately two to six times largerthan the area of the region 5124. After that, a difference of theposition between the searched region 5126 and the region 5124 in theimage 5122 is obtained as a motion vector 5127. The motion vector 5127represents motion of image data in the region 5124 in one frame period.Then, in order to generate an image illustrating the intermediate stateof motion, an image generation vector 5128 obtained by changing the sizeof the motion vector without a change in the direction thereof isgenerated, and image data included in the region 5126 of the image 5121is moved in accordance with the image generation vector 5128, so thatimage data in a region 5129 of the image 5123 is generated. Byperforming a series of processings on the entire region of the image5122, the image 5123 can be generated. Then, by sequentially displayingthe input image 5121, the generated image 5123, and the input image5122, moving images can be interpolated. Note that the position of anobject 5130 in the image is different (i.e., the object is moved)between the images 5121 and 5122. In the generated image 5123, theobject is located at the midpoint between the images 5121 and 5122. Bydisplaying such images, motion of moving images can be made smooth, andblur of moving images due to afterimages or the like can be reduced.

Note that the size of the image generation vector 5128 can be determinedin accordance with the display timing of the image 5123. In the exampleof FIG. 28A, since the display timing of the image 5123 is the midpoint(½) between the display timings of the images 5121 and 5122, the size ofthe image generation vector 5128 is half of that of the motion vector5127. Alternatively, for example, when the display timing is ⅓ betweenthe display timings of the images 5121 and 5122, the size of the imagegeneration vector 5128 can be ⅓, and when the display timing is ⅔between the display timings of the images 5121 and 5122, the size can be⅔.

Note that in the case where a new image is generated by moving aplurality of regions having different motion vectors in this manner, aportion where one region has already been moved to a region that is adestination for another region or a portion to which any region is notmoved is generated in some cases (i.e., overlap or blank occurs in somecases). For such portions, data can be compensated. As a method forcompensating an overlap portion, a method by which overlap data isaveraged; a method by which data is arranged in order of priorityaccording to the direction of motion vectors or the like, andhigh-priority data is used as data in a generation image; or a method bywhich one of color and brightness is arranged in order of priority andthe other thereof is averaged can be used, for example. As a method forcompensating a blank portion, a method by which image data of theportion of the image 5121 or the image 5122 is used as data in ageneration image without modification, a method by which image data ofthe portion of the image 5121 or the image 5122 is averaged, or the likecan be used. Then, the generated image 5123 is displayed in accordancewith the size of the image generation vector 5128, so that motion ofmoving images can be made smooth, and the decrease in quality of movingimages because of afterimages or the like due to hold driving can besuppressed.

In another example of the method for interpolating moving images in thisembodiment, as illustrated in FIG. 28B, when a generation image which isgenerated based on two input images which are temporally adjacent isdisplayed in a period after one image is displayed until the other imageis displayed, each display image is divided into a plurality ofsubimages to be displayed. Thus, moving images can be interpolated. Thiscase can have advantages of displaying a dark image at regular intervals(advantages when a display method is made closer to impulsive display)in addition to advantages of a shorter image display cycle. That is,blur of moving images due to afterimages or the like can be furtherreduced as compared to the case where the length of the image displaycycle is just made to half of that of the image input cycle. In theexample of FIG. 28B, “input” and “generation” can be similar to theprocessings in the example of FIG. 28A; therefore, description thereofis omitted. For “display” in the example of FIG. 28B, one input imageand/or one generation image can be divided into a plurality of subimagesto be displayed. Specifically, as illustrated in FIG. 28B, the image5121 is divided into subimages 5121 a and 5121 b and the subimages 5121a and 5121 b are sequentially displayed so as to make human eyesperceive that the image 5121 is displayed; the image 5123 is dividedinto subimages 5123 a and 5123 b and the subimages 5123 a and 5123 b aresequentially displayed so as to make human eyes perceive that the image5123 is displayed; and the image 5122 is divided into subimages 5122 aand 5122 b and the subimages 5122 a and 5122 b are sequentiallydisplayed so as to make human eyes perceive that the image 5122 isdisplayed. That is, the display method can be made closer to impulsivedisplay while the image perceived by human eyes is similar to that inthe example of FIG. 28A, so that blur of moving images due toafterimages or the like can be further reduced. Note that the number ofdivision of subimages is two in FIG. 28B; however, the number ofdivision of subimages is not limited to this and can be other numbers.Note that subimages are displayed at regular intervals (½) in FIG. 28B;however, timing of displaying subimages is not limited to this and canbe a variety of timings. For example, when timing of displaying darksubimages 5121 b, 5122 b, and 5123 b is made earlier (specifically,timing at ¼ to ½), the display method can be made much closer toimpulsive display, so that blur of moving images due to afterimages orthe like can be further reduced. Alternatively, when the timing ofdisplaying dark subimages is delayed (specifically, timing at ½ to ¾),the length of a period for displaying a bright image can be increased,so that display efficiency can be increased and power consumption can bereduced.

Another example of the method for interpolating moving images in thisembodiment is an example in which the shape of an object which is movedin an image is detected and different processings are performeddepending on the shape of the moving object. FIG. 28C illustratesdisplay timing as in the example of FIG. 28B and the case where movingcharacters (also referred to as scrolling texts, subtitles, captions, orthe like) are displayed. Note that since terms “input” and “generation”may be similar to those in FIG. 28B, they are not illustrated in FIG.28C. The amount of blur of moving images by hold driving variesdepending on properties of a moving object in some cases. In particular,blur is recognized remarkably when characters are moved in many cases.This is because eyes track moving characters to read the characters, sothat hold blur easily occur. Further, since characters have clearoutlines in many cases, blur due to hold blur is further emphasized insome cases. That is, determining whether an object which is moved in animage is a character and performing special processing when the objectis the character are effective in reducing hold blur. Specifically, whenedge detection, pattern detection, and/or the like are/is performed onan object which is moved in an image and the object is determined to bea character, motion compensation is performed even on subimagesgenerated by division of one image so that an intermediate state ofmotion is displayed. Thus, motion can be made smooth. In the case wherethe object is determined not to be a character, when subimages aregenerated by division of one image as illustrated in FIG. 28B, thesubimages can be displayed without a change in the position of themoving object. The example of FIG. 28C illustrates the case where aregion 5131 determined to be characters is moved upward, and theposition of the region 5131 is different between the subimages 5121 aand 5121 b. In a similar manner, the position of the region 5131 isdifferent between the subimages 5123 a and 5123 b, and between thesubimages 5122 a and 5122 b. Thus, motion of characters for which holdblur is particularly easily recognized can be made smoother than that bynormal motion compensation frame rate doubling, so that blur of movingimages due to afterimages or the like can be further reduced.

Embodiment 9

A semiconductor device can be used in a variety of electronic devices(including game machines). Examples of electronic devices are atelevision set (also referred to as a television or a televisionreceiver), a monitor of a computer or the like, a camera such as adigital camera or a digital video camera, a digital photo frame, amobile phone handset (also referred to as a mobile phone or a mobilephone device), a portable game machine, a portable information terminal,an audio reproducing device, a large game machine such as a pinballmachine, and the like.

FIG. 29A illustrates an example of a television set. In a television set9600, a display portion 9603 is incorporated in a housing 9601. Thedisplay portion 9603 can display images. Here, the housing 9601 issupported by a stand 9605.

The television set 9600 can be operated with an operation switch of thehousing 9601 or a separate remote controller 9610. Channels and volumecan be controlled with operation keys 9609 of the remote controller9610, so that images displayed on the display portion 9603 can becontrolled. Further, the remote controller 9610 may be provided with adisplay portion 9607 for displaying data output from the remotecontroller 9610.

Note that the television set 9600 includes a receiver, a modem, and thelike. With the receiver, general television broadcastings can bereceived. Further, when the television set is connected to a wire orwireless communication network through the modem, one-way (from atransmitter to a receiver) or two-way (between a transmitter and areceiver or between receivers) data communication can be performed.

FIG. 29B illustrates an example of a digital photo frame. For example,in a digital photo frame 9700, a display portion 9703 is incorporated ina housing 9701. The display portion 9703 can display a variety ofimages. For example, the display portion 9703 can display data of animage photographed with a digital camera or the like and can function ina manner similar to that of a normal photo frame.

Note that the digital photo frame 9700 includes an operation portion, anexternal connection terminal (e.g., a USB terminal or a terminal whichcan be connected to a variety of cables such as USB cables), a recordingmedium insertion portion, and the like. Although these components may beprovided on a surface on which the display portion is provided, it ispreferable to provide them on a side surface or a back surface becausethe design of the digital photo frame is improved. For example, a memorywhich stores data of an image photographed with a digital camera isinserted in the recording medium insertion portion of the digital photoframe, so that the image data can be transferred and displayed on thedisplay portion 9703.

Alternatively, the digital photo frame 9700 may transmit and receivedata wirelessly. Through wireless communication, desired image data canbe transferred and displayed.

FIG. 30A is a portable game machine, which includes two housings 9881and 9891 connected to each other with a joint portion 9893 so that theportable game machine can be opened or folded. A display portion 9882and a display portion 9883 are incorporated in the housing 9881 and thehousing 9891, respectively. In addition, the portable game machineillustrated in FIG. 30A further includes a speaker portion 9884, arecording medium insertion portion 9886, an LED lamp 9890, input means(operation keys 9885, a connection terminal 9887, a sensor 9888 (havinga function of measuring force, displacement, position, speed,acceleration, angular velocity, rotation number, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radial ray,flow rate, humidity, gradient, vibration, smell, or infrared ray), and amicrophone 9889), and the like. Needless to say, the structure of theportable game machine is not limited to the above and other structuresprovided with at least a semiconductor device may be employed. Theportable game machine can include other accessories as appropriate. Theportable game machine illustrated in FIG. 30A has a function of readinga program or data stored in a recording medium to display it on thedisplay portion, and a function of sharing information with anotherportable game machine through wireless communication. Note that thefunction of the portable game machine illustrated in FIG. 30A is notlimited to those described above, and the portable game machine can havea variety of functions.

FIG. 30B illustrates an example of a slot machine, which is a large gamemachine. In a slot machine 9900, a display portion 9903 is incorporatedin a housing 9901. In addition, the slot machine 9900 further includesan operation means such as a start lever or a stop switch, a coin slot,a speaker, and the like. Needless to say, the structure of the slotmachine 9900 is not limited to the above and other structures providedwith at least a semiconductor device may be employed. The slot machine9900 can include other accessories as appropriate.

FIG. 31A illustrates an example of a mobile phone. A mobile phone 1000includes a display portion 1002 incorporated in a housing 1001, anoperation button 1003, an external connection port 1004, a speaker 1005,a microphone 1006, and the like.

In the mobile phone 1000 illustrated in FIG. 31A, data can be input whena person touches the display portion 1002 with his/her finger or thelike. In addition, operations such as making calls and composing mailscan be performed when a person touches the display portion 1002 withhis/her finger or the like.

The display portion 1002 has mainly three screen modes. The first modeis a display mode mainly for displaying images. The second mode is aninput mode mainly for inputting data such as text. The third mode is adisplay-and-input mode in which two modes of the display mode and theinput mode are combined.

For example, in the case of making a call or composing a mail, a textinput mode mainly for inputting text is selected for the display portion1002 so that text displayed on a screen can be input. In this case, itis preferable to display a keyboard or number buttons on substantiallyall the area of the screen of the display portion 1002.

By providing a detection device including a sensor for detectinginclination, such as a gyroscope or an acceleration sensor, inside themobile phone 1000, display on the screen of the display portion 1002 canbe automatically changed by determining the orientation of the mobilephone 1000 (whether the mobile phone 1000 is placed horizontally orvertically).

Further, the screen modes are changed by touching the display portion1002 or operating the operation button 1003 of the housing 1001.Alternatively, the screen modes may be changed depending on the kind ofan image displayed on the display portion 1002. For example, when asignal of an image displayed on the display portion is a signal ofmoving image data, the screen mode is changed into the display mode.When the signal is a signal of text data, the screen mode is changedinto the input mode.

Further, in the input mode, when input by touching the display portion1002 is not performed for a certain period while a signal detected bythe optical sensor in the display portion 1002 is detected, the screenmode may be controlled so as to be changed from the input mode into thedisplay mode.

The display portion 1002 can function also as an image sensor. Forexample, the image of a palm print, a fingerprint, or the like is takenwhen the display portion 1002 is touched with the palm or the finger, sothat authentication can be performed. Further, by using a backlightwhich emits near-infrared light or a sensing light source which emitsnear-infrared light in the display portion, the image of a finger vein,a palm vein, or the like can be taken.

FIG. 31B illustrates an example of a mobile phone. The mobile phone inFIG. 31B includes a display device 9410 in a housing 9411, which has adisplay portion 9412 and operation buttons 9413, and a communicationdevice 9400 in a housing 9401, which has operation buttons 9402, anexternal input terminal 9403, a microphone 9404, a speaker 9405, and alight-emitting portion 9406 which emits light when a phone call isreceived. The display device 9410 which has a display function can bedetached from or attached to the communication device 9400 which has aphone function, in two directions represented by arrows. Thus, thedisplay device 9410 and the communication device 9400 can be attached toeach other along their short sides or long sides. Alternatively, in thecase where only the display function is needed, the display device 9410can be detached from the communication device 9400 and used alone.Images or input data can be transmitted and received by wireless or wirecommunication between the communication device 9400 and the displaydevice 9410 each having a rechargeable battery.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

This application is based on Japanese Patent Application serial no.2009-184323 filed with Japan Patent Office on Aug. 7, 2009, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: a pixel portion including a firstthin film transistor over a substrate; and a driver circuit portionincluding a second thin film transistor over the substrate, the firstthin film transistor comprising: a first gate electrode layer; a firstgate insulating layer over the first gate electrode layer; a firstsemiconductor layer over the first gate insulating layer; and a firstsource electrode layer and a first drain electrode layer over the firstsemiconductor layer, the second thin film transistor comprising: asecond gate electrode layer; a second gate insulating layer over thesecond gate electrode layer; a second semiconductor layer over thesecond gate insulating layer; and a second source electrode layer and asecond drain electrode layer over the second semiconductor layer,wherein each of the first gate electrode layer, the first gateinsulating layer, the first semiconductor layer, the first sourceelectrode layer, and the first drain electrode layer has alight-transmitting property, wherein a material of the first gateelectrode layer is different from a material of the second gateelectrode layer, wherein resistance of the second gate electrode layeris lower than that of the first gate electrode layer, wherein a materialof the first source electrode layer and the first drain electrode layeris different from a material of the second source electrode layer andthe second drain electrode layer, and wherein resistance of the secondsource electrode layer and the second drain electrode layer is lowerthan that of the first source electrode layer and the first drainelectrode layer.
 2. The semiconductor device according to claim 1,wherein the second source electrode layer and the second drain electrodelayer contain a metal material.
 3. The semiconductor device according toclaim 1, further comprising a capacitor portion over the substrate,wherein the capacitor portion includes a capacitor wiring and acapacitor electrode overlapping with the capacitor wiring, and whereineach of the capacitor wiring and the capacitor electrode has alight-transmitting property.
 4. The semiconductor device according toclaim 1, wherein the semiconductor device further includes an insulatinglayer over the second semiconductor layer, and wherein a conductivelayer which is provided over the insulating layer and overlaps with thesecond semiconductor layer.
 5. The semiconductor device according toclaim 4, wherein a material of the conductive layer is different fromthe material of the first gate electrode layer, and wherein resistanceof the conductive layer is lower than that of the first gate electrodelayer.
 6. The semiconductor device according to claim 1, whereinresistance of a first region in the second semiconductor layeroverlapping with the second source electrode layer and the second drainelectrode layer is lower than that of a second region of the secondsemiconductor layer which is other than the first region.
 7. Asemiconductor device comprising: a pixel portion including a first thinfilm transistor over a substrate; and a driver circuit portion includinga second thin film transistor over the substrate, the first thin filmtransistor comprising: a first gate electrode layer; a first gateinsulating layer over the first gate electrode layer; a firstsemiconductor layer over the first gate insulating layer; and a firstsource electrode layer and a first drain electrode layer over the firstsemiconductor layer, the second thin film transistor comprising: asecond gate electrode layer; a second gate insulating layer over thesecond gate electrode layer; a second semiconductor layer over thesecond gate insulating layer; and a second source electrode layer and asecond drain electrode layer over the second semiconductor layer,wherein each of the first gate electrode layer, the first gateinsulating layer, the first semiconductor layer, the first sourceelectrode layer, and the first drain electrode layer has alight-transmitting property, wherein a material of the first gateelectrode layer is different from a material of the second gateelectrode layer, wherein resistance of the second gate electrode layeris lower than that of the first gate electrode layer, wherein a materialof the first source electrode layer and the first drain electrode layeris different from a material of the second source electrode layer andthe second drain electrode layer, wherein resistance of the secondsource electrode layer and the second drain electrode layer is lowerthan that of the first source electrode layer and the first drainelectrode layer, and wherein the first semiconductor layer includes anoxide semiconductor.
 8. The semiconductor device according to claim 7,wherein the second source electrode layer and the second drain electrodelayer contain a metal material.
 9. The semiconductor device according toclaim 7, further comprising a capacitor portion over the substrate,wherein the capacitor portion includes a capacitor wiring and acapacitor electrode overlapping with the capacitor wiring, and whereineach of the capacitor wiring and the capacitor electrode has alight-transmitting property.
 10. The semiconductor device according toclaim 7, wherein the semiconductor device further includes an insulatinglayer over the second semiconductor layer, and wherein a conductivelayer which is provided over the insulating layer and overlaps with thesecond semiconductor layer.
 11. The semiconductor device according toclaim 10, wherein a material of the conductive layer is different fromthe material of the first gate electrode layer, and wherein resistanceof the conductive layer is lower than that of the first gate electrodelayer.
 12. The semiconductor device according to claim 7, whereinresistance of a first region in the second semiconductor layeroverlapping with the second source electrode layer and the second drainelectrode layer is lower than that of a second region of the secondsemiconductor layer which is other than the first region.
 13. Asemiconductor device comprising: a pixel portion including a first thinfilm transistor over a substrate; and a driver circuit portion includinga second thin film transistor over the substrate, the first thin filmtransistor comprising: a first gate electrode layer; a first gateinsulating layer over the first gate electrode layer; a firstsemiconductor layer over the first gate insulating layer; and a firstsource electrode layer and a first drain electrode layer over the firstsemiconductor layer, the second thin film transistor comprising: asecond gate electrode layer; a second gate insulating layer over thesecond gate electrode layer; a second semiconductor layer over thesecond gate insulating layer; and a second source electrode layer and asecond drain electrode layer over the second semiconductor layer,wherein each of the first gate electrode layer, the first gateinsulating layer, the first semiconductor layer, the first sourceelectrode layer, and the first drain electrode layer has alight-transmitting property, wherein a material of the first gateelectrode layer is different from a material of the second gateelectrode layer, wherein resistance of the second gate electrode layeris lower than that of the first gate electrode layer, wherein a materialof the first source electrode layer and the first drain electrode layeris different from a material of the second source electrode layer andthe second drain electrode layer, wherein resistance of the secondsource electrode layer and the second drain electrode layer is lowerthan that of the first source electrode layer and the first drainelectrode layer, and wherein the second semiconductor layer includes anoxide semiconductor.
 14. The semiconductor device according to claim 13,wherein the second source electrode layer and the second drain electrodelayer contain a metal material.
 15. The semiconductor device accordingto claim 13, further comprising a capacitor portion over the substrate,wherein the capacitor portion includes a capacitor wiring and acapacitor electrode overlapping with the capacitor wiring, and whereineach of the capacitor wiring and the capacitor electrode has alight-transmitting property.
 16. The semiconductor device according toclaim 13, wherein the semiconductor device further includes aninsulating layer over the second semiconductor layer, and wherein aconductive layer which is provided over the insulating layer andoverlaps with the second semiconductor layer.
 17. The semiconductordevice according to claim 16, wherein a material of the conductive layeris different from the material of the first gate electrode layer, andwherein resistance of the conductive layer is lower than that of thefirst gate electrode layer.
 18. The semiconductor device according toclaim 13, wherein resistance of a first region in the secondsemiconductor layer overlapping with the second source electrode layerand the second drain electrode layer is lower than that of a secondregion of the second semiconductor layer which is other than the firstregion.
 19. A method for manufacturing a semiconductor device comprisinga pixel portion including a first thin film transistor and a drivercircuit portion including a second thin film transistor, comprising thesteps of: forming a first light-transmitting conductive film over asubstrate; forming a second conductive film over the firstlight-transmitting conductive film, wherein resistance of the secondconductive film is lower than that of the first light-transmittingconductive film; forming a first resist mask including a first regionand a second region whose thickness is smaller than that of the firstregion over the second conductive film; etching with the use of thefirst resist mask so that the second conductive film and part of thefirst light-transmitting conductive film in the pixel portion, and partof the second conductive film and part of the first light-transmittingconductive film in the driver circuit portion are removed, therebyforming a first gate electrode layer of the first thin film transistorand a second gate electrode layer of the second thin film transistor;forming a first gate insulating film over the first gate electrode layerand a second gate insulating film over the second gate electrode layer;forming a first semiconductor layer over the first gate insulating filmand a second semiconductor layer over the second gate insulating film;forming a third light-transmitting conductive film over the firstsemiconductor layer and the second semiconductor layer; forming a fourthconductive film over the third light-transmitting conductive film,wherein resistance of the fourth conductive film is lower than that ofthe third light-transmitting conductive film; forming a second resistmask including a third region and a fourth region whose thickness issmaller than that of the third region over the fourth conductive film;and etching with the use of the second resist mask so that part of thethird light-transmitting conductive film and the fourth conductive filmand in the pixel portion, and part of the fourth conductive film andpart of the third light-transmitting conductive film in the drivercircuit portion are removed, thereby forming a first source electrodelayer and a first drain electrode layer over the first gate insulatingfilm, and a second source electrode layer and a second drain electrodelayer over the second gate insulating film.
 20. The method formanufacturing a semiconductor device according to claim 19, wherein thefirst resist mask and the second resist mask are formed using amulti-tone mask.
 21. The method for manufacturing a semiconductordevice, according to claim 19, further comprising the steps of: formingan insulating layer over the first source electrode layer, the firstdrain electrode layer, the second source electrode layer and the seconddrain electrode layer; forming a fifth light-transmitting conductivefilm over the insulating layer; forming a sixth conductive film over thefifth light-transmitting conductive film, wherein resistance of thesixth conductive film is lower than that of the fifth light-transmittingconductive film; forming a third resist mask including a fifth regionand a sixth region whose thickness is smaller than that of the fifthregion over the sixth conductive film; and etching with the use of thethird resist mask so that the sixth conductive film and part of thefifth light-transmitting conductive film in the pixel portion, and partof the fifth light-transmitting conductive film and part of the sixthconductive film in the driver circuit portion are removed therebyforming a first conductive layer overlapping with a first channelformation region of the first thin film transistor and a secondconductive layer overlapping with a second channel formation region ofthe second thin film transistor.
 22. The method for manufacturing asemiconductor device, according to claim 21, wherein the third resistmask is formed using a multi-tone mask.